It's fine, it happened a long time ago. But to put it all simply she fell in love with a demon, and Earth is the only place where angels and demons can both go. And I couldn't find her since she was with the demon. She ended up finding me.

I could tell you the whole story, but I don't know how much sense it would make... [And she really doesn't like talking about Lucifer's son, even if he isn't here.]