Why does my ex boyfriend get quiet when I talk about other guys?

Just to be clear, my ex dumped me. We spent about a year not talking and then we became friends again. We were best friends before we started dating so I'm happy we're friends.

I have zero feelings for him anymore.

I figured we could talk like any other friends talk, right? But I'm having problems with a guy that I'm half dating and I was whining about it. He was lively before but just kind of went quiet and tried to change the subject.

I doubt he still has feelings for me but why would he do this?
Why does my ex boyfriend get quiet when I talk about other guys?
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