Why would a guy suddenly be self conscious?

I’ve been talking to an admittedly shy, reserved guy for a while. In the beginning he was very chatty, made jokes, and even sent some dirtier texts. In person he’s only ever kissed me and nothing more. My humor hasn’t changed but he’s suddenly taking my joking serious. Especially if I imply he’s anything other than innocent and quiet. Yet, he shoot’s down anything nice I say about him. Even if it’s not a direct compliment.
Why would a guy suddenly be self conscious?
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