It's the Principle of the Thing

Some characters have such strong principles that they will seek vengeance for a wrong done to someone else even when that someone else knows it isn't serious.

Daniel has a semi-tragic event happen to him that's not a huge deal. However, in Emma's mind, it is a grave crime and justice must be served! Daniel is Emma's best friend/close relative/significant other, and whoever has barely hurt him must pay! Any sane person would shrug his shoulders (as Daniel usually does in these situations) and accept that you can't win 'em all, but to Emma, it's the principle of the thing.

If it's Daniel and not Emma who goes nuts, then he is merely obeying the Law of Disproportionate Response. Often goes hand-to-hand with the Frivolous Lawsuit.

Live Action TV

 * Malcolm in The Middle had a particularly vicious one: Dewey wins a hole-in-one at the 18th hole of a mini-golf course; according to the rules, that entitles the golfer to a free game. But according to Dewey, the owner of the place didn't give him the free game because the bell didn't ring. In Dewey's mind, and as he explains in as many variations as possible to his dad, it's not a big deal; getting the hole-in-one was good enough for him. But Hal acts as if to say, "This is a huge deal! I know how upset you are about losing this. That golf course is gonna pay for ripping you off!" Hal's comments are along the lines of, "You have to feel this way, Dewey! Justice must be served!" Eventually, Hilarity Ensues, and it's all Hal's fault.
 * Basil Fawlty and House are well known for their constant need to make every problem far worse than necessary because they want to prove a point or won't swallow their pride.
 * Frasier took this to epic levels. Once he and Niles pulled into a parking garage, realized they had forgotten something, and turned around to exit. The attendant charges them $2.00 for the one minute they were in there (The fee is $2.00 for every fraction of a half-hour). Frasier adamantly refuses to pay, refuses to let Niles pay for him, and even refuses to let another driver behind him pay. Somehow Frasier believed that he was "standing up" against this "injustice", when obviously he was being petulant and downright rude. Finally Frasier relents and agrees to pay the $2.00, whereupon the attendant tells him that the fee is now $4.00 because he stayed over a half hour protesting. Frasier floors the gas and charges through the gate.
 * Niles, of course, sneaks back to make the $4.00 debt good to the attendant, and ends up having to write out a sizable check to pay for the destroyed gate (probably hundreds of times as much as the original $2.00 charge).
 * During Season 5 of The Amazing Race, Colin became furious at a cab driver in Tanzania for making them lose their lead by driving on a spare tire and having a blowout. He refused to pay the full $100 fare, instead offering only $50. Despite the urging of Christie, the other teams, and the airport staff to pay the fare, he continued to refuse, getting the police involved, and coming perilously close to becoming the first contestant to be arrested during the race. He finally paid the fare just because he didn't want to miss his flight.

Film

 * In The Big Lebowski, the dynamic between the Dude and Walter epitomizes the trope, with bad things endlessly happening to the chronically laid back and mellowed out Dude, and slightly demented Vietnam veteran Walter convinces him time and again to go on insane and half-cocked quests for revenge and reparation.
 * MARK IT ZERO!
 * Robin's response, word-for-word, in Robin Hood Men in Tights when Achoo tries to stop him from fighting Little John over a bridge toll by hopping over the two-foot-wide creek and pointing out "This ain't exactly the Mississippi here!"
 * Outright mocked by Porter in Payback. No, it's not the principle of the thing, and don't ask him again, you're going to make him get all misty.

Theatre

 * In The Golden Apple, Ulysses and the boys are playing baseball when Helen, whom they have sworn to protect, flies away with Paris to Rhododendron, but they consider it nothing to get angry over. Menelaus and the old men of Angel's Roost are outraged at their lack of honor, and remind them of their oath. Ulysses accuses them of "distorting the principle of the thing," but the old men echo this phrase back at him, and urge vengeance on Rhododendron.

Webcomics

 * In Megatokyo, after several rounds of BSODs, Yuki says that Tohya called her a monster, but that it's nothing compared to what she (Yuki) did to her . Kobayashi, on the other hand, invokes this trope briefly before Yuki stops him.

Western Animation

 * In the Doug Episode "Doug Takes the Case," The Rich Bitch Beebe Bluff's radio is stolen, and her father wants the criminal found, even though she could easily afford a new one, because "Its the Principle of The Thing."
 * Reversed by Daffy Duck in "My Little Duckaroo": "After all, it isn't the principle of the thing, it's the money."
 * A Garfield and Friends episode featured a police officer giving Jon a speeding ticket. Consideirng the size of the speed limit sign, Jon refused to pay and told Garfield and Odie it was not for the money but for the principle. Garfield told Odie that, whenever somebody says something like that, it's for the money.