Why would a dumper get upset that the dumpee moved on?

chx000
So my ex dumped me, and I was upset about it. I tried to get her back at the time and she said no. She said she never loved me or felt that much chemistry for after almost a year of dating.

I went NC for over 2 months. However, I then saw her at a function as we are in the same circle/community. The encounter was brief, awkward, but cordial enough.

I then saw her a second time and it was friendly enough. However, some dude was hitting on her. My ex seemed to enjoy the attention and I was upset about. But, what could I do? She was my ex. So, I took the high road and was nice to both them.

So then I started dating around (nothing serious) and my ex found out about it.

The next time I saw the ex, I approached her in a friendly manner, asked about her and her family, and she responded very rudely and cold towards me.

I am confused: she dumped me, she said we could remain friends.

What gives?

If I knew that staying away from other women would have given me a shot back with my ex, then I would have acted accordingly.

But don't y'all think that it was up to my ex to either contact me or give me some sort of signal that she was interested in reconciliation? I can't read minds, and I did not believe that being cordial was enough to signify the desire for a reconciliation.

And if she was not interested in a reconciliation and was ok with remaining friends, then why the heck get upset that I moved on?

Could it be all ego and the fact that I apparently moved on before her?

Either way, the whole thing makes me sad because I would have loved to have gotten back together with her. Short of that, it would have been ok to at least remained civil and friendly with her.

What did I do/not do that I should have/should not have?

Thanks.
Updates
+1 y
So, how should I act around her now? Risk being rude and ignore her, or continue to say hi with a smile, like I have been doing. I feel kind of silly doing that though, but maybe killing with kindness is best?
Why would a dumper get upset that the dumpee moved on?
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