I agree with KTTunstall888 (another great artist, by the way). It's about having this incredible person with you and then for reasons you can't remember anymore, you just let them go. It seems like it's maybe about a relationship that ended over an argument that at the time the singer couldn't fogive his lover for. And now, when getting back together seems impossible, he's going over everything he wishes he'd done differently.
I agree with KTTunstall888 (another great artist, by the way). It's about having this incredible person with you and then for reasons you can't remember anymore, you just let them go. It seems like it's maybe about a relationship that ended over an argument that at the time the singer couldn't fogive his lover for. And now, when getting back together seems impossible, he's going over everything he wishes he'd done differently.