Why did he start to get upset when I teased him?

At work we have a friendly atmosphere, everyone teases everyone and I’m close to this guy in particular. One day at work everyone was teasing him for being dressed up that day. It was all fun and games and when I started to tease a little bit he said he was starting to get mad. His mood only started to change when I made a joke and I was wondering why this may be. We always tease each other

Why did he start to get upset when I teased him?
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