After two years together how can an ex just jump into a new relationship within weeks of the breakup?

Anonymous

It ended mostly amicably despite caring a lot about each other (so I thought). How can people move on so quickly?

Updates
10 mo
Thanks for all of the opinions. To all of those who are saying he’d already checked out before we broke up (I think you’re probably right) but then why would he be nicer to me afterwards and want to meet up with me again as soon as I went no contact?
After two years together how can an ex just jump into a new relationship within weeks of the breakup?
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