On the very first page of Freak the Mighty, we learn that Max earned the name "Kicker" in daycare. According to Max, he would kick "anyone who dared to touch [him]." Without hesitation, Max would kick people who would hug him. Even as a child, Max could recognize that the hugs were "phony."
One reason why Max may have decided to react in this manner could be related to his unstable upbringing. He started day care when his grandma and grandpa took him in as their own - later, we learn that Max's father is in prison and his mother is dead.
From Max's perspective, the hugs were a symbol of pity. People felt sorry for him due to his situation, but Max didn't accept their pity.
The only person he never kicked in daycare, we learn, is Freak. In the first chapter, Max argues that this was due to the fact that Freak used crutches. However, if you trace the relationship between Max and Freak throughout the book, you notice that Freak is the one person who doesn't pity Max. Instead, he builds him to reach his potential, boosts his confidence, and becomes his best friend.