This is my take on it. I think the song is about a woman he used to be with and when he saw her at the subway he wanted to say something to her, but didn't. It was a chance meeting that may have seemed like divine intervention, but neither of them made a move. In his mind during that moment he spent a lifetime with her. I find this song sad but funny at the same time b/c my ex-bf didn't quite get the lyrics and thought it represented out relationship, but he meant it in a good way. Ironically the song is somewhat true for us because I dumped him and he can never be with me again.