You were in a love relationship of some kind, may be a marriage. It may have lasted a long time or only a short time but in either case, you had a lot invested in it. It is hard to let go of things and ideas that you may have been really attached to.
Most people do not enter into relationships lightly. They go into them hoping for something that will last a long time and just want love. You have dreams of the way that things could be. You have an idea of the way things should be. For some reason it doesn't happen. The bubble bursts on your idea and then you are left hanging on. It made such good and almost perfect sense at the time and then it is gone. You have to adjust to the idea that what you saw isn't there anymore if it was at all.
Was it love or just the idea of being in love? The lines get so confusing sometimes. Was the love ever there? It is very likely that it was at one point. The only problem was that it wasn't permanent. The two of you became incredibly important parts of each other. You get used to having certain things or people attached to you and when they are gone your mind may have trouble adjusting.
There are those who have lost limbs who still feel a phantom itch in the appendage that is now gone. There are still brain cells that are telling you that what isn't there itches and there isn't a thing you can do about it unless you retrain your brain.
The same thing is true for those who were intimately attached to someone, whether it was romantic or platonic in nature. If those people are removed for some reason, those parts of your brain that had grown accustomed to that loved one being there will have to adjust. While your brain is adjusting to the change, you are left thinking about them almost against your will.
If you are frustrated because you keep asking yourself, “Why do I want my ex back?” don't get too distraught over it. It may help you to get some advice on how to get over a relationship from someone who has been there or who understands and has helped others. It is only natural that you will have trouble with it and “want my ex back.” Give it some time, get some help, and get distracted and in time you will no longer be asking, “Why do I want my ex back?”