I think this song is about the guy being so sad that the girl got over him and took his heart and hers with her when she left. Obviously his heart was broken unevenly, and she took the bigger part of it. She has now moved on, and the guy actually treats her right and this guy is still crying and wondering what went wrong. I think that happens to everyone. When someone leaves you, you wonder why? Some people don't even see it coming, I think that happened in this song as well.
I love this song!
The singer's voice is amazing and all of the song is perfect for my "Sad, Slow, and Depressing" playlist. :)