Guys, How did I get this so wrong?

I asked every guy I know for advice on this one. They all said the same, he's shy and really likes you... yet today I see him all over this pretty girl, and It wasn't to get my attention. He is into her. There's been a massive change in dynamics.

How does someone go from being so intimidated by me, that he struggles to talk. Stares from afar and musters everything he can to smile at me. To flipping the coin completely and pursuing another girl?

I could go further into it, but basically he was so confident around every other girl but me, and had bags of nerves every time he approached me. If I approached him, he'd freak out. I'm so confused on how someone can just 360 like that.
Guys, How did I get this so wrong?
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