This is an answer I cannot give. There are only theories and these are few and not yet scientifically confirmed. Perhaps they will someday but it will require more open communication between wannabes and the medical community than what is the case today.
What I can say though is that it is not a conscious choice from my side. To me a one-legged life just feels more "normal" than a two-legged one. I know most will consider this impossible but I do not know in what other way to describe the feeling.
However, most wannabes can remember an incident in their early youth when they met or saw an amputee. Possibly this is a triggering event. The reason behind it all is unknown to me though. My personal theory is that there is something else, so far unidentified, behind it all. I do not believe that you are born a wannabe but perhaps some people are born with the possibility to become wannabes if the right stimulus (not necessarily a single event) is present at the appropriate time. If this presumably biological factor exists it could very well be possible for it to be the culprit behind other syndromes as well.
If the reason is of biological or purely psychological nature remains to be discovered.
Sometimes the syndrome is classified together with disorders where the affected individual will try anything to make his or her body look "better". Here you find eating disorders like anorexia and bulimia. Some (including myself) see a difference however. An anorectic person will se him- or herself as fat even when being nothing but skin and bones. A true wannabe’s need on the other hand is satisfied and at peace once he or she is an amputee.
The book by Furth & Smith discusses the psychological aspects of the syndrome and in time I will include a narrative here as well.