Doing post-mortems are just part of the game. Not everyone likes to do them and its ok to decline any invitation to " go over the game ". Its rude however to just walk away from someone who is talking to you imo. Just tell them you dont do post-mortems and then walk away, is that really so hard to do ?
Post mortem: Helpful or not?
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I don't usually like to discuss the game with my opponent either. I would rather analyze the game either by myself, or with someone else. After a long rated game I don't analyze right away, instead I like to play some nice, casual, blitz chess to cool down before the next round.