Why is he acting like this?

So, me and my fiancée wedding day is approaching really fast. As him and I were finalizing the decision of our wedding, "he said do you really want (my friends name) in the wedding?" I said yea, of course she's my best friend! Then he started complaining about how he didn't like her because she was the only one out of my friends that wasn't married or in a relationship. It was funny to me because only two of his friends are single, but I wasn't complaining. But I would admit that I noticed that he never liked her. Whenever she came over, he would always find some excuse to leave, and she treated him with respect and not overstepping boundaries. But with my other friends? He treats them like gold. Why is that?

Why is he acting like this?
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