The movie is not really good and...

that is in huge part because Dexter is actually just a self-centered *beep* for most of it. I am surprised that many women like this movie, with the male main character just doing whatever serves him best and then basically relying on Emma to get him out of the hole he dug for himself. Before the point Emma dies, there is not even any real character development for Dexter. He starts out this self-centered womanizer and stays like that until he cannot afford to be this kind of guy anymore. In my view, he is using Emma, just the way he uses everyone else.
In another topic someone brought up 500 days of summer. A movie where the relationship the movie revolves around has a similar dynamic. However, that movie succeeds where One Day fails because it never makes any pretense about the central relationship being symmetrical.