The topic of ex's

I'm curious why some people can't let go of certain things that have happened in the past. I, myself, have experienced hurt in relationships, but i've learnt that in time, your heart heals. Each relationship has it's own value, a lesson learnt and a spot in my heart. But I believe that in time, your heart heals, especially when you have entered a new relationship. This is when the door to your heart opens for your new "someone". I believe that when you enter a new relationship, you have put down the past and are ready to start something new and fresh with your new somebody. I have friends who are in a relationships and can get really upset when they talk about their ex's. Their emotions fluctuate alot when they talk about their ex's. They talk about how happy they were, what he/she use to do, what they miss and the list continues. Does that mean that they are not over their ex? Is it fair on their current partner? I don't really understand. What does it mean when an ex is getting married in August, but totally refuses to hear about you or meet you because you are a "sensitive topic"? When I heard he was getting married, I was very happy for him. I said to my friend that he must be very happy and in love his current girlfriend but my friend said "No, not really. There are still somethings he can't put down." I can't seem to understand this concept. How can someone marry someone if you can't put down the past? But he obviously can.

I always feel that it's fate that two people can be together. Whether it works out is another matter but the friendship and bond will always be there. I feel lucky to be able to stay friends with that someone who I got to know really well. Who I could share my happy and sad times. It's ashame to have them out of your life but I guess thats not often the case.