What prevents you from being friends?

I broke up with my girlfriend a year ago. Al
Although it wasn't a very amicable breakup. She seems much happier now and I've moved on with someone else. But recently when we ran into one another because we do have mutual friends. She was polite but yet very cold towards me. When I asked her why she responded with "I don't stay friends with exes and there's no reason from me and you to catch up"

If everything is in the past why can't we be at least hold a conversation?
What prevents you from being friends?
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