Why would he have sex with someone else. When he's friends say he likes me?

We are both in college, he's friends had been annoying me for years. About this, this guy always acts nervous around me and not with others. Then I find out from a trusted friend, that he was having sex. Why would he act like he likes me? But fuck someone else?

Why would he have sex with someone else. When he's friends say he likes me?
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