I've been there. I've been there for over thirty years, in fact.
Why does it hurt? It hurts because of the loss. It's that simple....Well,...uhm...Almost that simple.

If you love someone, the pain of seeing him/her harmed in any way, or not be happy would make you more miserable than distancing yourself.

If you love someone you would want what's best for them. That ain't no "They say." That's just the way it is.

It'll hurt like the dickens for a while. You'll always remember, but you'll get over it for the most part.