If a guy doesn't say hi to you anymore?

This guy was flirty with me for a few weeks during the summer, but he never asked me out even though I gave him hints that I was free and single and that we should do something together.

Then I didn't hear from him for like 2 weeks after school ended, and I saw him on campus today and he was walking past me. Looked at me, then just looked away and kept walking (wtf>?).

Why was he so flirty and friendly during the summer and then now he doesn't even say hi to me..
If a guy doesn't say hi to you anymore?
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