Why do I get nervous and shy around my crush?

I'm very expressive with my close friends. Just yesterday, two of my friends and I were talking about him a lot. They know I have a huge crush on him but keep my secret. I was describing him in a very elaborated manner.

We were in the library and he passed by. He's friendly and say hi to others. He said hi to us and got into a small conversation about classes. He brought his ancient Egyptian book he likes reading about too. My only reply to him was a ''Hi. Oh, nice book''.

I sounded so awkward (I didn't even smile), briefly looked at him once and then kept looking at the floor. Why do I act so different around him? So much to say about him with my friends and in my journal but nothing much when he came.

Updates
5 mo
I went from talkative to practically silent around him, sooo awkward.
Why do I get nervous and shy around my crush?
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