Why does he want me now that I'm with someone else?

psychological standpoint?.....The guy enjoyed the attention but attention from one girl wasn't good enough. Now that he sees you're no longer available, it's sort of a challenge for him to see if he can get you thus making his ego swell. Unfortunately, (most) guys always want what we can't have and even if we do get what we want, we're going to try and see if we can get better...until we learn the hard way :)