Disproportionate Reward

It's human nature to focus on the negative, to overreact to slights and swear Disproportionate Retribution on the perpetrator far in excess of the crime.

Then there are times when characters are kind to each other, going out of their way to be nice and help someone else for no other reason than Good Feels Good thanks to the benevolent part of human nature.

They'll wish they hadn't.

The receiver will proceed to make it his life's work to repay this minor favor done with no intent for reward with a lifetime of devoted friendship and service. It might be because the receiver has never been helped or shown kindness before, or having a rather strange set of personal ethics; whatever the case he considers this small service to be the most selflessly wonderful thing done for him, ever.

Cue them having a life debt for the hero, becoming a True Companions -- helpful, annoying, or overwhelming) that the heroes can't shoo off because the receiver's technically good, too.

If the receiver's ethics are so skewed that his idea of good are truly at odds with the good of the hero, then you have the Poisonous Friend. Expect Stop Helping Me to be uttered sometime.

See also Does Not Know How to Say Thanks for a Trope that can lead to this.

Contrast Dude Where's My Reward where the reward is smaller (instead of bigger) than what you have deserved, and Androcles Lion and Character Witness, who give a pretty much proportionate reward.

Anime and Manga

 * In The Cat Returns, Haru rescues a cat (actually Prince Lune of the Cat Kingdom) from being hit by a truck; the Cat Kingdom, specifically the King thereof, insists on repaying her with such things as boatloads of lacrosse sticks, cattails in her family's yard, boxes of mice, and even marriage to the Prince.
 * Justified in that it's objects being given in repayment for saving a life.
 * The last one is also justified in that Haru, in a fit of anger, snaps at the cat messenger and tells him that she didn't like any of the cat-related gifts she was given. The messenger is so saddened at them failing to please her that the faint, brief interest she shows when marriage to the prince is brought up is taken as a sign of approval.
 * Minato in Hyakko follows Torako around, trying to help her, after Torako does her a favor. Hilarity Ensues.
 * Ryoga's Mark of the Gods in Ranma Half.

Film
"Mrs. Potato Head: "You saved them? Oh, they're so adorable! Let's adopt them!"
 * In The Phantom Menace, Jar Jar swears declares himself the "humble servant" of Qui-Gon for pushing him out of the way of a trampling alien.
 * In Toy Story 2, Mr. Potato Head immediately regrets saving some alien toys from falling out of a moving car - mainly because they tend to do nothing but follow him around and say "You have saved our lives. We are eternally grateful."

Alien Toys: "Daddy!"

Mr. Potato Head: "Ohhhhhhhh...""


 * They do end up returning the favor in Toy Story 3....
 * Mystery Team. The protagonists deal with murderous drug dealers, strippers, disgusting toilets and drinking dog urine... for a dime.

Literature

 * Kreacher the house elf in Harry Potter and The Deathly Hallows. Hermione explains that house elves are people too (well, they have feelings too, okay?), so when Harry is kind to him and makes him felt valued, Kreacher bursts into tears, bodily teleports Mundungus Fletcher in for questioning, and cleans the Black house from top to bottom, providing them with delicious food. What specifically prompts his gratitude is being given a possession of "Master Regulus's". He probably never received gifts even from people who were nice to him.
 * Harry even points this out when he says that Grimmauld Place felt more like home now that Kreacher liked them.
 * Justified because from this point, Kreacher more or less understood that helping Harry is probably exactly what Regulus would want Kreacher to do.
 * The same goes for Dobby, who spends the rest of his life being grateful to Harry for freeing him from his former masters. This leads to him bringing Harry a crucial item needed to solve the Second Task in the fourth book, as well as helping to spy on Malfoy in the sixth and . Justified in that Harry really did give a shot at a much better life (working at Hogwarts is much nicer than working for the Malfoy family), but Dobby still goes overboard (he seriously considers throwing himself out of a tower if he doesn't fulfill Harry's request, he goes without sleep to spy on Malfoy, etc).
 * That much could be a house-elf thing, as they tend to overreact to the tiniest thing they do wrong and work themselves half to death to fulfil an order.
 * In The Diamont Chariot, Erast Fandorin considers saving the life and freedom of Yakuza Masahiro Shibata a very natural thing, yet he also manages to save his personal honor, prompting "Masa" to become his lifelong Battle Butler in return.
 * In the Star Wars Expanded Universe book I, Jedi, Corran Horn, under the pseudonym Keiran Halcyon at the time, was training with lightsabers and lightsaber training droids (you know, like the one that zapped Luke in A New Hope) with Mara Jade. Mara missed blocking some of the stun bolts, and Keiran caught them in midair with his hand, using his ability to absorb energy to nullify the damage. Later, Mara Jade show up in a Big Damn Heroes moment to pull Keiran's rear out of the fire when he got in over his head in a fight with the undead/ghost Dark Lord of the Sith Exar Kun and had his arm and some ribs broken, and a concussion. Mara simply said that she owed him a favor when he asked why. Keiran lampshades it by saying "If this is how you repay favors, I'll catch stun bolts for you any day."
 * Wookiees have "life-debt", which make the Disproportionate Reward a part of their code of honor. Worse, in some case, your children have to continue paying your debt. The example is Han Solo freeing Chewbacca from Star's End. And when, you consider the debt not to be paid?
 * It gets to point where it begins to seem like their just using it an excuse to keep the ever-popular Han around.

Live Action TV

 * In the Argentinian soap Los Roldán (remade as Los Reyes in Colombia and Los Sánchez in Mexico), a humble man finds a random old lady near the edge of a bridge crying for having been diagnosed with a terminal illness, and convinces her to not commit suicide; later the lady, who just happened to be one of the richest businesswoman in the country, hires him as the president of her company with a exorbitant salary, makes his family move to her lavish mansion, and essentially changes her will to benefit them all. In the beginning this seems like this trope, but it's later subverted
 * George Burns and Gracie Allen had an episode where Gracie agrees to host a wedding for the daughter of a woman who "did (her) a wonderful favor!" After all hilarity has ensued, George asks what this amazing favor was.
 * In a season one episode of Friends, Phoebe finds that her bank account has been accidentally credited with a large amount of money. She reports this to the bank, who confuses her message and interprets it as her ´´missing´´ this amount, doubling it! Feeling like she doesn't deserve it, she gives it to a homeless woman, who insists that she return the favour by buying her a can of soda. The soda contains a human thumb, which Phoebe reports to the company, and freaks out when she receives $10,000 in damages that she doesn't want and feels like she's entirely unentitled to.
 * She resolves her feelings by putting the money to good use.
 * Done in an episode of MASH where Hawkeye saves a marine who was shot in the neck. Granted, he did save the marine's life, the marine spends the rest of the episode fulfilling a life debt, constantly doing favors for Hawkeye such as scaring away people in the mess tent so Hawkeye could have a place to sit, and twirling Frank Burns around in the air to prevent him filing a report on some of the hijinxs going on at the 4077.
 * Don't forget, he smacked Hawkeye's ass at least once. So maybe there's something more going on there.
 * In one episode of Parker Lewis Can't Lose, Kubiak gives his last quarter to a guy on the street so he can make a phone call. At the end of the episode, the guy swept in and paid off Kubiak's $10,000 debt. Turns out he was a musician who was using the call to set up a multi-million dollar tour.

Mythology

 * The story of Saint Martin, who gave a freezing beggar half his cloak one night. It turns out the beggar was Jesus in disguise, who granted him Sainthood for his compassion.
 * In Greek mythology, the gods and goddesses were quite fond of pretending to be mortal peasants and seeing who would be kind enough to give them food or shelter. Basically, if you passed this test and showed basic human kindness, you'd get all kinds of riches and favors.

Theater

 * Cyrano De Bergerac: Deconstructed because Cyrano and Roxane had a very unusual set of personal ethics.
 * At Act I Scene VII, Ligniere begs Cyrano to let him sleep in his house to avoid an ambush. Cyrano it’s so enchanted with Ligniere’s gesture of Courtly Love to a Lady that he insists on fighting the One Hundred Thugs hired to punish Ligniere. Justified because Cyrano is a Gascon and they have a rather strange set of personal ethics… and because Cyrano is looking for a challenge and one hundred men are more or less that for him
 * At Act IV Scene VIII, Christian ask Roxane why she risked her life meeting him at the battlefield, she responds him that it was for his (really Cyrano’s) love letters. When Christian lampshades that were a few, inconsequent love – letters, Roxane answers him that those are Serious Business.

Video Games

 * Many quest-line based RPGs, especially MMORPGs. Help find lost friends? Here, have a few potions that can save more than a few lives. Retrieved a lost item from a cave? Take this piece of gear that is far more valuable than that item as an reward. Sometimes a distinct Gameplay and Story Segregation, but can also sometimes be justified.
 * Bodhan Feddic swears himself to Hawke's service in Dragon Age 2 after Hawke rescued Bodhan's son Sandal from a situation that he really didn't need rescuing from. Silly!Hawke lampshades this more than once.

Web Comics

 * In the world of Fox Tails, kitsunes pay back everything disproportionately, and saying Stop Helping Me is pretty much a deadly insult.
 * Invoked in Girl Genius: Zeetha points out that the Jagers have already helped Agatha enough for the one time she saved them. So they obviously have another motivation.

Western Animation

 * In one episode of Johnny Test, Gil dramatically volunteers to help rescue Johnny from a gang of super villains... because Johnny once helped him see that his shoelace was untied.
 * On The Looney Tunes Show, Granny told a story about how she saved the Eiffel Tower from being stolen by Nazis. As a reward, the French gave her the Eiffel Tower. The one in Paris is a fake.
 * One of the musical segments (entitled "Be Polite") involves Mac and Tosh repaying everyday acts of courtesy with eleborate gifts.

Other

 * A LOT of folktales work like this, with someone(usually the main character) doing some tiny good deed for someone or something who turns out to be a god or other such powerful being. The receiver of the good deed usually returns the kindness by giving their benefactor wealth, power, or happiness beyond their imagination.
 * Also common in Glurge. One example is the man who attends a funeral, only to see a completely empty funeral in the next parlor. He signs the logbook for the deceased. Later, he is contacted by her attorney, who informs him that her entire estate was to be divided amongst those who signed the log at her funeral.
 * In the case of folklore, a bit of it can be attributed to Values Dissonance - most of the services rewarded are either giving beggers (or beggers in disguise) food or shelter. In the days when the folktales took place, charity like that was more than just being nice; it was potentially saving someone from starving or dying in the elements.
 * See also Sacred Hospitality.
 * To elaborate on Values Dissonance -- money was quite rare for a long time and worth a lot, so not many people would part with some for a beggar and the beggar wouldn't be able to feed themselves regularly. There was also no social safety net, so they had to depend entirely on the rare charity of others.