I Didn't Tell You Because You'd Be Unhappy

A character decides to not reveal important information to another character because they don't want to ruin whatever it is the second character is doing. The reasoning for this is that this information will cause the second character to do something they would not want to do, but would feel emotionally obligated to do if they had this information.

The character who the secret is being kept from will almost always be upset when they find out about it.

A common motive for My Secret Pregnancy. See also Awful Truth. Sometimes also a case of I Want My Beloved to Be Happy, if revealing the secret would break up the Love Interest's current relationship.

Examples of I Didn't Tell You Because You'd Be Unhappy include:

Anime and Manga

In Dragon Ball, most of Goku's friends don't tell him about his were-monkey problem because they were afraid that he'd put two and two together and realize that he was probably the very monster that killed his (adoptive) grandfather. He eventually figures it out when he sees Vegeta transform, but he's too busy trying not to get killed to BSOD over it.
In Higurashi no Naku Koro ni, Rika not only doesn't tell Akasaka exactly why he'll regret not going back to Tokyo, in the manga, she actually cuts the cord of the telephone lines so he can't call the hospital his pregnant wife is at. It isn't until Ooishi finds him with Rika on a hill that Akasaka finds out his wife is dead. Rika gives this trope as the reason for not telling him, because she knew that he'd fall into such deep despair that he wouldn't be able to help her.
In Tiger and Bunny, Kotetsu has a problem with disclosing personal issues, even when they have major consequences for others, because he doesn't like worrying people and bothering them with his own problems. This ends up biting him in the ass on a grand scale in the second half of the series.
In Natsume Yuujinchou, this is the reason Natsume is reluctant to tell the Fujiwaras about his problems with youkai and ayakashi - They'd have to live with the fact that he is attacked by invisible monsters every day, and that there's very little they could do about it.

Film

In Shaun of the Dead, Shaun's mum doesn't tell him she's a Zombie Infectee because she didn't want him to worry. *sniffle* This isn't so much "you'd be unhappy" as "you'd be even more unhappy," though, under the circumstances.
In The Dark Knight, Alfred burns a letter from the recently-deceased Rachel, saying that she was going to marry Harvey Dent, in order to keep from further tormenting Bruce/Batman.
- And this is pretty much the justification for not making it public that Harvey Dent went crazy, killed several people and kidnapped Gordon's family, and that the ensuing standoff lead to his death. Of course, fancier words were used.

Literature

In Robin Hobb's Farseer and Tawny Man trilogies, Fitz never lets Burrich know he's still alive because Burrich is married to Fitz's former sweetheart Molly and he wants them to be happy. Burrich finds out about fifteen years later, and is understandably upset, but he dies before Molly finds out, and Molly marries Fitz.
In Kokoro by Natsume Soseki, Sensei hides the reason for his constant guilt from his wife so her innocence won't be tainted by the world's evils.
Played with in the Isaac Asimov short story "Liar!", one of several which eventually became the book I, Robot.

Live Action TV

Baywatch episode "Baywatch Down Under". Jake learns that his ex-wife didn't tell him she had become pregnant with his child before divorcing him, because she felt he needed to be free and didn't want him to feel obliged to stay with her.
In Supernatural Dean doesn't want Sam to know that he sold his soul to save him because it would make him unhappy. Sam finds out anyway.
- There's a fanfiction alternate-season-four where Zachariah edited Dean's memory hard at some point in the process of bringing him back, so while he's still Dean in all kinds of ways he has no particular emotional attachment to, for example, Sam or the Impala, and isn't likely to die for Sam again. Then Zachariah turns up and explains to him that, actually, Dean Winchester broke so badly in Hell that they couldn't bring him back, and he's just someone who was tortured so badly his own identity vanished, and Zachariah just refitted his soul so Sam would have a big brother to keep him stable. Dean eventually buys it, but doesn't tell Sam for exactly this reason.
- Actually this is just a thing Dean does.
In one episode of Curb Your Enthusiasm, Larry's mother dies while he is shooting a film in New York. No one in his family tells him this because it was actually her dying wish that they not bother him about it. Larry is understandably upset that his mother died, had a funeral, and everyone has moved on- all while he was filming in New York and only just found out about it.
- He calls her out on this when he meets her in the afterlife.
In the short-lived series The Cape, an astronaut goes on a Shuttle mission as his wife is about to give birth. She dies of complications during the birth, and Mission Control keeps this from the crew until they return to Earth.
Smallville. Too many times used to count.
This happens in an episode of Lie to Me: Lightman doesn't tell Torres about her boyfriend being located at the site of an accident because she'll be more useful to everybody if she stays and helps identify the culprit.
The longest thread in the history of alt.tv.buffy-v-slayer had the heading "Xander Lied!" It was about Xander not telling Buffy that Willow was re-trying Angel's re-ensoulment spell in the final episode of season 2.

Theater

In Sweeney Todd: The Demon Barber of Fleet Street, Mrs. Lovett uses this as justification for not telling Sweeney that his wife Lucy is still alive, but an insane, homeless prostitute.

Video Games

In Fire Emblem, Lord Uther has actually been beset with crippling illness for most of the game. Oswin is aware of the illness from the beginning, but doesn't tell main character Hector about it under Uther's orders. Uther knew that Hector would have left Eliwood and Lyn's party and would have spent the rest of his life regretting it if he were told.
Tales of Symphonia, Colette hides 1. What's happening to her as she's releasing the seals; 2. That she's supposed to die at the end of the Regeneration; and 3. That she's being crystallized by a disease. all to avoid her companions worrying about her. In short, she loves pulling this almost as much as she loves Lloyd. Raine also knows about the second one, but plays along with Colette's wishes as well.
Tifa of Final Fantasy VII avoids telling Cloud about what actually happened at Nibelheim. Considering the likely outcome of that knowledge it's understandable, but it really does cause a mess.