It felt to me that the guy left the girl because for him they're always fighting and the relationship never grow. They never changed, so he left. Now, he's a sort of regretting why he left her, but he can't go back now. The girl love him very much that she'll do everything for her, but he wants her to be happy and move on. Things changed. Even though it hurts and they still love each other, but not on the same way as before, they still have to live.