My answer to the question is because you love that person. Been here too.. When you start to love, you start to get attached. and those attachment espcially if dugay na, lisod au putlon. We can't say "buwag nami" and poof ur detached. You need time to cut those ties. Majority stares at the closed doors for too long, you would still yearn, you would still be concerned you still feel love and you will feel pain until such time you start to accept things and move forward then you'll see the door that has opened for you.
Remembering the memories you have with your x is really normal. I still remember the days me and my x are together, but I'm not inlove or attached to her anymore. Simple, it's a part of your life and like it or not, those memories are one of the factors why you became who you are right now. ^^