Why is it so hard to talk to an ex-boyfriend or ex-girlfriend?

It's hard to talk to an ex, when you still have some feelings there. Whether it be positive feelings (like still having love for them)or negative feelings (hating them for something they did to you, or hating the break up). It's best to work on your feelings, be on neutral ground if you wish to communicate with an ex.

because talking to them reminds us of past memories and fights and how much we cared about them and now it is all over. We also do get jealous

Because if a relationship did not work out, there is usually (not always) varying degrees of animosity and its difficult to know how to approach a interaction where people have broken apart.

When you talk to or see your ex-boyfriend or ex-girlfriend, all the memories whether they are good and bad comes into your brain. There is no couple on earth who had only bad memories in a relationship. At least some of the memories are always there that you need to re-live. Good memories are far stronger than bad ones. No matter how a couple underwent a breakup but if they ever loved each other there is also a small corner of softness still left for each other. That mixed feeling of desire and hatred make the person indecisive about interacting with her/his ex. The feeling makes you want her/him back and at the same time makes you want to ignore your ex.

Its hard because there are normally a lot of unanswered questions and emotions that have not been fully healed, it takes a lot of time to be able to speak to someone you once loved without it being pain full or confusing or frustrating, in short its hard because you have not fully healed yet

I've never been really in a relationship, but from what I've seen and observed, an ex-partner isn't just some person from the past. They're people, who once, became the most important people in our lives. Even after the relationship ends, the memories, experiences, and the baggage will still be there.

It's not easy walking out of a relationship, especially if there are some feelings still left. That could be a reason why is not easy to talk to an ex. It might bring back memories that hurt you.

Because you have a history with them. That history probably includes sharing feelings, intimacy, fights and so on. After a couple breaks up it's hard to talk to them because all the things you used to talk about, might be harder to touch on as just friends. Friends have a great connection and so does a couple, but they are still different.

Because they probably have had all the really good memories along with them despite breaking up. It will bring back all the memories of when they were together and it may also bring back the emotional sadness.

Usually, there are a lot of emotions attached to that person and all that you went through. It can be difficult to remember the past which often occurs when talking to an ex. Some people pass this as time goes by but some don't and that's just as valid and perfectly okay too!