Kathryn's completely right — you can't make somebody love you, and if you did love the girl, you wouldn't try to come between her and the man that's making her happy.
What Would Happen If I Tried To Make Her Love Me Instead Of Him?
- You'd embarrass yourself. If I had feelings for someone who wasn't interested in me, I'd back off and leave them alone, because it's degrading, undignified and pathetic to chase someone who doesn't want you. You can't help who you love, but if they don't love you back, save yourself the embarrassment.
- You'll cause the girl a lot of stress. I've been in this situation before, several times, and it's horrible. I felt guilty, worried, uncomfortable, and avoided that person as much as possible. Put yourself in her shoes — how would you feel if somebody that you didn't love, or even like, kept pestering and/or emotionally blackmailing you?
- She will tell her boyfriend, and he won't be happy. If you keep trying to make her love you — or keep trying to ask her out on dates, or talk to her all the time on Facebook — she'll get stressed and he'll be angry. They might even argue about what should be done (she might want to take a more sensitive approach, whereas he might want her to tell you straight) and that would make them both miserable. If you loved the girl, you wouldn't want her to be miserable.
Trying to break up a couple is morally wrong. If you really love her, just be her friend, but don't try to get too close. Keep your distance and be pleasant to her, and don't make a move unless they break up.
Don't try to split them up — just because she's not with him, it doesn't mean she'll want to be with you, especially if you're the cause of the break-up.
I feel so strongly about this because I know how unpleasant it can be. I've been in tears over situations like this, so if you truly love the girl, just leave her alone until she's single. Even if/when that happens, you can't make her love you.
What to do if she loves someone else? 
