Why would a guy always say hi or wave when he sees me in hallways when he is walking with his friend?

I always see him walking with his friends, but when we meet in hallways he always says hi. I also notice that when he notices me from far away he waves. We barely have talked and we r just acquaintances.
Why would a guy always say hi or wave when he sees me in hallways when he is walking with his friend?
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