"You've always been kind to me, Mary, whether I deserved it or not," Severus whispered again, his voice becoming progressively more quiet the more choked up he became. "Even when I wasn't kind to you.
"I know that probably doesn't sound like much to you, since that's what your mother has always told you to do that with anybody. But it's the fact that you didn't make an exception for me -- because of how I look, or what House I'm in, or the fact that I'm not popular, or whatever the excuse of the week is ... that's what makes all the difference.
"Other people find it all too easy to make an exception when it comes to me, but you gave me a chance when most people wouldn't. That's why." Granted, he was talking about more than just this year. He was talking about his first foray into fifth year, as well, but he couldn't tell her that. Perhaps someday, however, he would be able to.