"Harry," Severus said, gently reaching out to hold his hand. "We have talked about this before. There are certain people who cannot know about this. Your father and I had a very tumultuous relationship during our time at Hogwarts. We hated each other with a fiery passion and were the worst of enemies. If I favor you as soon as you start Hogwarts, certain people in powerful positions will begin to suspect something and they will hurt you or me to get to the other."