Guys, Why the sudden change of behaviour?

Was on a course recently and one of the people on the course started trying to engage in conversation, and saying hi more often. I could tell he liked me, cause the second day he was a bit nervous and shy when he said hi.

Then suddenly the afternoon comes and he avoids me completely. I did nothing wrong, and to be quiet frank, I was nice to everyone. Why did he suddenly was so ignorant I’ll never understand. He’s my age too, and I didn’t show interest, I was just friendly as I always am.
Guys, Why the sudden change of behaviour?
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