Why will a guy who is in a room when he sees me come in the room he leaves right away why?

There could be several reasons why a guy leaves a room when you enter. He might feel shy or anxious about interacting with you, or he could be trying to avoid an awkward situation. It's also possible that he simply doesn't want to engage at that moment, or he may have other reasons unrelated to you. Understanding his behavior might require more context about your relationship and his personality.