Why is unrequited love painful?

I agree with you earth,unrequited love is indeed painful.It really hurts when we don't receive any love or recognition from the person that we love,and it hurts more to know if the person has a significant other in his/her life.But that's unconditional love...loving without expecting any return.If he/she will love us in return..then great but if not,then it's okey.All we have to do is keep on loving..after all one of our purposes in life is to love...

I also think that it hurts because its a form of rejection. We've set ourselves up to be hurt. In some cases we are attracted to emotionally unavailable men/women. In other cases we are really attracted to someone that may be a close friend. Either way, feeling like they don't want you hurts.