What does this mean.. please read on?

I hear people talk about how they talk about as they get to know their bf's better their bfs seem to become more attractive. But whenever i get into a relationship with even the cutest guy with the greatest personality you could imagine i always seem to think he gets even more unattractive and i pick out his flaws and then end up dumping him... my mom says i'm too attentive and observant, but i don't think so.... please help