I think he just changed his mind without any particular reason. You said he could have any girl he wanted, so maybe he met someone over there that reminded him of his ex. I'm not saying that's what happened, I'm just saying that it could be a valid theory, that's all.But his feelings have changed quite a lot..and I don't know why..
Here. Have a BBT gif to cheer you up.
Never fails to put a smile on my face. ^_^