"Because I love you." And he knows that's not the answer she was looking for, but there isn't any other one he can give.

"I love you because you're a strong person no matter what you think, because you're good and kind and compassionate and so many other people wouldn't be after what you've gone through, because I admire your courage...I can't even explain all the reasons why I love you. But I'm not leaving you. You would have to kick me out for me to not stay with you."