A character decides that he or she would do all they can to see the person they like be happy rather than doing their best to end up with that person. Thus, the character acts against his or her own interests, often supporting relationships between their beloved and another person.
If a straight guy chooses to do this, expect one of two things to occur: some variation of "If You Ever Do Anything To Hurt Her," or the girl to turn around and decide she wants to be with him anyway, as a reward for his virtue. Unfortunately, seeing as the latter reduces the woman to a prize to be won for good behavior, it doesn't always make the audience happy...
Also see The Paolo.
Contrast Murder The Hypotenuse. Compare/contrast Love Makes You Evil.http://tvtropes.org/pmwiki/pmwiki.php/Main/IWantMyBelovedToBeHappy