Why is this male coworker friendly to me in front of others but unfriendly when we are alone?

When he passes me by and greets me he looks to the side annoyed, with everyone else he makes good eye contact, he always tries to keep conversations short, makes excuses to leave early and never tries to make small talk with me despite being chatty and charming with everyone else. However around others he is very nice to me and gives me his best smile etc. We never had any arguments, I am not ugly or overweight so why does he act like this? Do men really treat unattractive women like this?

Why is this male coworker friendly to me in front of others but unfriendly when we are alone?
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