Why are you a Multiple?
There is probably no one good reason for that. Some people developed unique strategies to cope with their life. Some people may have been just born that way. Others had a talent for channeling in passing spirits, did so, and it became a permanent arrangement eventually. This answer can't really encompass all the reasons, because each person is unique. The best way to find out how one particular system developed is to ask the people in that system. Maybe they've given it considerable thought.