I'm not proud of the fact that I have a lot of ex-girlfriends in my past, and I'm not proud to admit that I may add a few more to that number before my dating days are over, but I'm least proud of the fact that very, very, very few of these women that I've dated are on the "I still want to talk to you list". In fact that list has really only one name on it, the rest of my interactions with ex's are nothing more than awkward, stale, “forced to be cordial” conversations that usually end quickly.
To me relationships require a lot of commitment for them to work. They take communication and trust, balance and sacrifice, not to mention passion, and when a relationship ends, so too usually does the desire to continue doing the work to maintain any of these things, with that person.
In truth, it's not that I didn't try to stay friendly with more of my ex's, it's that I really didn't want to. Once I've turned a corner, I usually stay on that path that I've chosen, and seldom if ever look back. I really think that maintaining a relationship that has ended leads to "arrested development", or to put it a differently: It keeps you living in the past.
I guess what I'm saying is that in the long run, if the wounds heal and time erases the damaged feeling, I suppose it is possible to be friends with an ex, I just don't think it's probable. There are sooooo many variables that had to happen to put you together in the first place, that now when you're apart, it would take that same lightning to strike twice for you to truly remain friends.
I would have to say that's sad. I'm actually still friends with 90% of my ex's and glad to say so. Don't think of it as living in the past ... it's more like turning the page to a different chapter. There's no reason you can't stay friends with these people. Obviously you liked them enough at one time to associate with them (and closely, I might add) ... and they're still the same people. True, the first few weeks or months may still be a bit awkward, but with a little effort on both parts to ease the awkwardness, this soon passes, and you can now associate with this "cool" person on a whole new level. True, there are some asses that no matter what you do intend to make life hell for you, but they are few and far between. It's a definite sign of maturity when you can move on to a whole new level with an ex without any bitterness or grudges. Try it sometime. Why burn bridges?