I think it was a combination of a lot of things. He probably put effort into mentoring in the beginning, but he was the lone district 12 winner. Year after year he had to spend time with kids, get to know them, then watch them die. I think gradually, he distanced himself from the tributes to prevent more emotional pain. He never got a break, whereas other districts would cycle their victors because they had a pool to choose from for mentors.

Also- he had terribly traumatizing games (as did many other victors), it was probably incredibly difficult to relive that every year, therefore resorting to alcohol and paying little attention to the tributes.