I used to have a teacher that was always mean, bitter, etc. I would find out what their hobbies were, what they did for spare time, and talk to them about it. Then after they started becoming more friendly to me, I started joking with them and they really turned the corner and were in a better mood.

When they started to get in a bad mood again, I would ask "Do you need a hug" and that would always bring a smile or laughter from them.