There's a difference between being with someone, and being for someone. I can say that I am in love with him, and that I want to be with him because with him, I feel complete and I feel whole. But saying that would make me selfish because my feelings for him would still lead to what I want and what I feel, and not his.

When you really love someone, you don't just want to be together with him, you want to be a better person for him. You want to quit your vices, to take your priorities seriously, to aim for high marks in school, to be a more pleasant person. You want to change your old, unpleasant ways. You want to start of as a new and improved you. You want to be a better person and that's all because of him. Because he inspires you to be a better person. You are changing not for the better of yourself and mankind, but for him.

I am not in love with you because "you complete me", I am in love with you because "You make me want to be a better man (woman in my case)." To be in love is not to sacrifice to be with him; to be in love is to sacrifice FOR him.