The entire song is like a huge Fuck you, but more to himself than to her. In the chorus he pretty much sums everything up. Why is it that she reminds him of what he really is? He's stating that he is the cheating, the playing along, the toying with. He's as much worth as she makes him feel. And he is a second best and maybe not even that. And maybe his persona is reduced to that. To being someone's "sometimes" person, the guaranteed one to call along when lonely.