It's a good guess, and one I have heard several other people (me included) suggest.

Certainly I do not believe that Snape's regret was Dumbledore's sole reason fro trusting him. There has to be more to it than that.

Do you know the most surprising thing about divorce? It doesn't actually kill you, like a bullet to the heart or a head-on car wreck. It should. When someone you've promised to cherish till death do you part says, "I never loved you," it should kill you instantly.

- Under the Tuscan Sun