Why do you want to be friends with your ex?
You want to be friends with someone because they care about you and want what's best for you. They are there to support you and help you through rough times.
When your ex broke up with you (this question is for dumpees) your ex most likely treated you poorly and wasn't there for your emotional needs.
So why do so many people want to be friends with their exes?
This life is so weird. You are left with only two choices: to be friends or to be strangers. To heal you need to be strangers, but this means that you will lose all connection to the other person. I do feel upset that I will lose the friendship that I had with my ex (don't hate me please all who read this). I guess we have to be strangers...