Why did he get into a new relationship so quickly?

My ex and I split just under three months ago. Two months after the breakup (no contact), he was in a new relationship. With a girl, I might add, that he'd known the entire time we were together. I broke up with him because of broken trust, he had no respect for me and I had reason to think he was up to no good.

He is the sort of guy, looking back, who has always needed to have a girlfriend. He's never really been single. I can't help but feel that his ability to move on so quickly invalidated our entire relationship and it was a massive waste of my time, or is this a rebound?
Why did he get into a new relationship so quickly?
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