Why my ex can't look at me or talk to me?

I am curious about this situation. I had a boyfriend who has broken up with me four months ago. He has moved to his hometown and I have stayed in the city where I do my studies. We haven't seen each other since that event and we only have exchange few messages. The thing is that we have some mutual friends and one of his friends is my roommate. So, these friends and my ex came to visit us and I they all greeted me except him. He was avoiding to even look at me all the time and we bearly exchange some words. I find that what he was doing was impolite and childish. I would like to hear more opinions about this subject because these encounters really interest me.
Thank you
Why my ex can't look at me or talk to me?
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