Why is my ex mad I moved on when he dumped me?

How fast, and with who, and how did you do your “moving on?”

Not enough detail, for an accurate answer.

If you moved fast, it’s logical for an ex to suspect that the new person was already “involved” with you, while you claimed to be loyal.

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If you “moved on” with someone who was a mutual supposed friend, your ex is likely to feel betrayed on two sides of his life.

If you “moved on” ostentatiously, so as to more or less flaunt your replacement for him, or to show him and everyone else the two of you know, that you think so little of him that he could be replaced by literally anyone, any ex would logically get angry at the intentional insult.

If he’s in pain that the two of you broke up, which most grown-up people are, and you appear to be unaffected by the breakup at all, again, your “moving on comfortably” would imply that he was meaningless to you, and again, anger would be logical, albeit unfortunate.

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I see the leading responses calling your ex various rude names or accusing him of childish immaturity, but I’ve seen enough different people in the world to know better than to jump to snotty conclusions, especially without evidence.