This discussion may contain spoilers!
When talking with Kenny near the fireplace, somewhere during the conversation you are given the option to say "Lee saved me". After saying that, Kenny will say one of two things. He will either say "I knew it, I knew he would. That guy had a fire in him sure as hell." Then he will say "Hell". Then this is where he either says "Lee and I had our differences" or "Lee was a great guy". What makes him say one or the other, is it based on the decisions you made in Season 1? Btw, I quoted these by memory so don't quote me quoting them. I tried to be as accurate as possible, but that's the general idea.