Whats wrong with talking to an ex?

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I'm curious as to why so many people on here are so against talking/being friends with an ex. Whats the big deal? I'm still best friends with one of my exes and see nothing wrong with it. My fiance knows and we all hang out often and have a great time. I know everyone has their reasons but I just think it's kind of ridiculous that just because two people dated they should have to cut off all communication, especially seeing that some people just discover they don't do well in a relationship but are great as friends. Obviously there are exceptions (cheating, bad crowd, etc.) and I don't mean this in a rude way, I'm just genuinely curious. What are your thoughts on the matter?