Played straight with Kaworu. When he realized Shinji and Asuka had gotten together he was delighted because Shinji was finally happy, even if it was with another person.
Subverted with Rei. She feels Shinji is happy now that he and Asuka are patching their relationship up; but due to strong jealousy, she refuses to realize Asuka cares for him and interferes as much as possible.
Rei falls in love with both Shinji and Asuka, but she refuses to try to have a relationship with them because she does not want to interfere with their love bond:
"I... love you both," the blunette said softly. She raised her eyes with a noticeable struggle. "Shinji, you showed me care and compassion when no one else ever had, not even the Commander. Asuka, you helped free me from the drugs that clouded my mind when you had no reason to do anything but dislike me. And you both came for me and saved me in this battle. I... I look at you and I know I love you both." "Rei, we..." Shinji trailed off. What could he say? He looked over at Asuka. She shrugged too, at an equal loss. Rei waved her hands defensively at their shocked expressions. "No! I do not want to intrude! I... I will never try to... intersplice myself in your bond! It is too beautiful to disrupt! But... I wanted you both to know. I understand what this feeling is now. It is love. I will protect that feeling, and you. I will not allow any harm to come to you or your bond if I can ever do anything about it."
Subverted with their female classmates. They have guessed what something is happening between them and Asuka has kind of claimed Shinji, but they will not stop trying to seduce him.
Averted with Kurumi, who for a long time is after Shinji, even though he makes it clear that he does not love her and marries someone else. After babysitting Akiko, though, she stops hitting on him.
Played straight with Rei, who even encourages Asuka to open up and hook up with Shinji.
Shinji And Warhammer 40 K: Shinji did not make anything about Asuka seeing Kaworu because it made her happy and that the most important thing to him. Even after Kaworu had revealed his true self, when Asuka asked him if he was not going to try and convince her to not be tempted, Shinji still told her: "Decide what you want." Subverted when he said he was not afraid to lose Asuka because she was his.
In TSA The Amazing Spider Man After Gwen and Peter break up due to Peter keeping secrets from her, she starts dating Johnny Storm of the Fantastic Four. which Peter accepts despite wanting to be with her, because Gwen is with a guy she likes, who can be honest with her.
In New Feelings, Jade (originally The Mole sent by her brother Peridot to kill Garnet to get back at Pearl) decides to move on after Pearl gets engaged. Unfortunately, this doesn't end well for her.
James Potter does this in the James Potter Chronicles series by Mean St. When Lily and Snape have a major fight in fourth year, it's James who convinces Lily to talk to Snape and give him another chance. When asked why he did this by Lily, James explains that her happiness is more important to him than his feud with Snape; as much as he hates Snape and would prefer it if Lily stopped being friends with him, he can tell that Lily is miserable and wants her to be happy again, even if that means helping her to make up with Snape.
A Discussed Trope in A Growing Affection: Sakura asks Naruto to leave Hinata to be with her, claiming Hinata will be fine because she would want Naruto to happy. Naruto tells Sakura he knows she is not in love with him, because she did this with Sasuke, but she is not doing it with him now.
In the Star Trek (2009) fanfic Safe and Sound, Christine Chapel, who has feelings for Spock, pulls one of these towards the end of the story, upon realizing that his heart belongs with Uhura.
In The Tangled Princess Bride, Prince Walter to Rapunzel, especially since it's implied that this King Edward was the one from Enchanted and takes place after that film's events.
In the Avengers/Hunger Games fic Fire and Lightning, Peeta eventually does this by falsely telling Katniss he lied about being in love with her, so that she doesn't feel guilty pursuing a relationship with another boy.
Escape From The Hokage's Hat: Subverted in that Sakura just sees Naruto as a friend. Naruto has a crush on Sakura. However, after seeing Naruto bleeding, broken, and dying from fulfilling his promise to bring Sasuke back, she practically BEGS him to find someone else. For all the times she says she "loves" Sasuke, he ends up committing treason and abandons her, while Naruto is always there for her due to his crush; it would be unfair to Naruto for him to do so much for her while all he gets is Unrequited Love in return.
Fai has feelings for Kurogane but decides to help R!Syaoran with his relationship troubles because being with R!Syaoran makes Kurogane genuinely happy.
Sakura breaks down in tears when Syaoran tells her that he loves Kurogane more than her, but is happy that Syaoran will not be alone when he has to continue to dimension-travel to not cause a paradox.
This is the subject with one Touhoufankoma, in which Yukari encourages Rinnosuke to pursue a romance with Reimu, despite having a crush on him herself. The twist with this being is that, at the end, she is shown to have outlivedthem and her heart is shattered.
At the end of the Kim Possible retelling of Bram Stoker's Dracula, Drakigo, Ron comes to accept that Kim is more in love with Shego then she is with him and allows her to be with Shego. But he only lets Kim go after making Shego swear that she'll look after Kim or else.
In The Power of Love and Magic Charlie Weasley is in love with Harry, but remains supportive of his relationship with Charlie's brother Bill... right up until Bill becomes engaged to Fleur Delacour, without telling Harry it's over. At this point, Charlie proceeds to kick Bill's ass.
Saetwo's Story: In practically the same breath that Uloobu admits to Saetwo that he has a crush on her, he tells her that Echae cares far more about her than anyone else, and that she should make up with him after their earlier fight.
Cody: If someone like her is willing to break down whatever lone wall of solitude that they've built up in order to give you a chance to be with them, you take it AND YOU DON'T LET GO! And...you don't let go. (sigh) I should have never let her go...
In the Pokémon fanfic Olivine Romance: Erika harbors a lesbian crush for Jasmine. Realizing Jasmine's heterosexual preference and the impossibility of reciprocation, Erika conspires to give Jasmine happiness by setting her up in a relationship with Volkner. Only in a moment of extreme torment does Erika reveal her feelings for Jasmine with a kiss. Jasmine turns her down gently.
In the Katekyō Hitman Reborn! story Sehnsucht, the main character's former incarnation, Cervello Cynthia, was in love with Daemon Spade. However, he was in love with Elena, her adoptive sister, so Cynthia refrained from ever telling him her feelings out of respect for their relationship.
A Senseless Sacrifice example occurs in the backstory An Alternate Keitaro Urashima. Keitaro confessed his feelings to Mutsumi, who turns him down because she thinks he belongs with Naru. Instead Keitaro ends up dating someone else and can't stand Naru once he meets her.
In I'll die before I lose you again Sif, who's in unrequited love with Thor, uses a magic sword given to her by Amora to save the life of a mortally wounded Jane, sacrificing her own in the process.
Seen in Skyhold Academy, a Dragon Age: Inquisition AU, but the trope doesn't appear for a while because Varric does a masterful job of hiding his feelings. Not only is he fairly sure that his Love Interest wants someone else who (he thinks) would be better for her, but she's also extremely sick; he outright tells his confidante that as long as she's alive and happy, that's enough for him.
The RWBY/The HobbitFusion FicRise of a Star Knight has an interesting version. Throughout the story, Jaune has been talking to a spirit version of Pyrrha, eventually said to be (partially) a manifestation of his guilt over her (apparent) death. It's revealed that Pyrrha is actually alive, and that spirit Pyrrha is actually a piece of her aura that was transferred during their kiss before Pyrrha apparent death (part of his aura also ended up in her, which saved her from that death, and from torture). At the end, it's revealed that, while it is part of her, it's a copy, having a mind of its own, and it sacrifices itself to save Jaune so that he can be happy with the living Pyrrha.
In By Right of Conquest Hermione, who's madly in love with Harry, refuses to even consider a relationship because she'd prefer a child-free marriage and not having a large family would devastate him.
To Hell and Back (Arrowverse): Barry Allen is in love with Iris West, but has yet to confess his feelings to her. He was aware she would most likely already be in a relationship by the time he returned to Central City — and she was, with Eddie Thawne. As a result, he encourages their relationship and resists any and all temptations of revealing how he really feels. Even learning that he and Iris will one day be married doesn't sway him from this stance.
In Unexpected Surprise, Chat Noir moves to America in order to give Ladybug a chance with the other guy she loves and broke up with him over. If only he knew who the other guy was...
Miraculous: Marinette's baby: While being with Adrien would make her happy, Marinette breaks it off with him after she hears his argument with his father, believing that she would be taking away his future otherwise.
A platonic, deconstructed example occurs in the Temeraire fanfic Black Wings, Black Sails- Laurence and Temeraire's internal conflicts are driven by their adamant refusal to believe they have anything to offer each other, and will most definitely be happier with someone else. Laurence fully expects to be hanged for his crimes before long- he can't bear the thought of Temeraire wasting his potential with a non-Aviator-turned-pirate, and eventually either grieving over losing Laurence for real, or getting executed himself. Temeraire, on the other hand, is insecure about everything that sets him apart from his Western cohorts, believes Laurence must have left him because he's not a good enough dragon, and is overprotective of Dayes in Laurence's place.
Broken Souls Mend Hearts: When Killian doubts his possible skills as a father, he starts distancing himself because he doesnt think hes good enough for Emma, Henry, and the baby.
In Hellsister Trilogy, Brainiac 5 is heartbroken when his long-time flame Supergirl falls for Dev-Em. Nevertheless, he notes Kara looks happy and Dev-Em looks good for her, so he has no option but resign himself to it.
Brainiac 5: The woman I've wanted for most of my life is upstairs now, making love with a man who has nothing of what I have to offer, and I have nothing of what he has to offer. The worst part of it is, I can't even hate him. He's probably good for Kara. The battle's done, to the victor belong the spoils.
The One To Make It Stay: Thanks to her closer relationship with Luka, Marinette has several pictures of him on her phone during the "Oblivio" incident; as a result, she doesn't assume that she and Adrien are dating. Adrien is hurt by this, but accepts it. While he confesses his feelings to her before she releases the Miraculous Cure, he makes clear that this is purely for his own sake, and that she's not obligated to return his feelings. Unfortunately, the same cannot be said of Chat Noir normally, and when Alya posts a heavily edited video of his confession to the Ladyblog, his refusal to accept no for an answer causes plenty of problems...
White Sheep (RWBY): A double version with the Jaune/Yang/Ruby Sibling Triangle. Yang is terrified that she is stealing her sister Ruby's friend and possible love interest (and this has happened before, so it's not an unreasonable fear), and keeps trying to think of ways to let Jaune down gently. Ruby, on the other hand, realizes that this is the first time Yang has been genuinely interested in a boy (all of Yang's previous "dates" were just friends who suddenly tried to get under Yang's shirt), and tries to encourage Jaune to go after Yang. It says something that Ruby is more proactive about it than Yang; Yang keeps thinking she needs to do something, but ends up enjoying herself too much and never discouraging Jaune at all. Jaune and Yang officially start dating at the dance, when Yang thought he was going with Ruby but he actually went alone because he knew Yang was busy.