Why would a guy say he's shy?

Anonymous
This really outgoing guy started getting quiet around me recently.

Once I made a comment that he stands with us but doesn't talk like he thinks he's cooler than us. Just as a joke, and he got kinda weird and blurts out "i'm shy ok". He repeated it a couple times a while after.

Would a guy act outgoing with a girl for months then get shy around her?

Would a guy just say he's shy to get a girl to shut up?

What reasons could there be for a guy to say he's shy?
Why would a guy say he's shy?
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