Well, they did rather detest each other. Not unlike yourself and Mr. Malfoy. And then, your father did something Snape could never forgive.” “What?” “He saved his life.” “What?” “Yes . . .” said Dumbledore dreamily. “Funny, the way people’s minds work, isn’t it? Professor Snape couldn’t bear being in your father’s debt. . . . I do believe he worked so hard to protect you this year because he felt that would make him and your father even. Then he could go back to hating your father’s memory in peace. . 

J.K. Rowling