Why would an ex do this?

I ran into an ex at a party. I caught him looking st me a few times. The girl he brought and myself were in the kitchen so we just had small talk. I said nothing about who I was or bad mouthed him in any way. He then tried to get the girl away from me. As they were leaving I told her it was nice to meet her and wished her a merry Christmas. The ex wouldn't even look at me but I wished him a happy holiday anyway. He refused to acknowledge me. This drama happened a year ago. Is it too much to ask to be friendly?
Why would an ex do this?
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