how would you react?

Then I will try to forget him as he loves someone else. There are so many people in the world, I will find someone I love and he loves me, too.

Guess it would depend on the situation, for one. If I was in a relationship with that person, I would feel hurt, devastated, and betrayed to start with. If it was someone that I wasn't in a relationship, I would be sad about it and feeling a bit let down with life in general (not the person), but I would be happy for them in the end. What would I do? Nothing. What can you do? People feel what they're going to feel, and if there's anything that I've learned, it's that you can't change that. If it were me in a relationship and they fell for someone else, I would be beyond hurt but I wouldn't fight for someone who didn't want me. If it were just someone close that I loved, I'd let them go. If they were meant for me then I'll get my chance someday, and if not...well, then it's not meant to be and the best I can do is be happy for them.