Guys, Why he acts shy/nervous/avoidant?

A guy I've gotten to know over the last few months is showing signs he likes me/attracted to me such as always running his hand through his hair while looking at me, staring at me from across the room, etc. When his female friends talk and make jokes about their bodies or body parts (like boobs) he laughs, but when I do it, he gets quiet and starts avoid me. Why is this? Are some guys really that shy? Or is he just uncomfortable? I'm not sure if he is just shy or uncomfortable.

Guys, Why he acts shy/nervous/avoidant?
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