Why guys suddenly pull away?

This guy and I have been conversing daily for a month. He even joked about us sharing a room. We are good friends, but I am not sure if he likes me.
That is, it was during a trip with our friends, it was the first time I meet him "he lives across the country." We spoke for five minutes after he left his buddies to say hello. He did not even look at me or say goodbye for the remainder of the journey. He behaved as though we are complete strangers.

What does this mean?

Why guys suddenly pull away?
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