This matter of choice ... do you really feel that this applies here? You yourself said, that poly chose you. For me, it sounds like asking "Why do you chose to love?" or "Why do you chose to have a realtionship with other human beings?" I personally believe that I can not choose what I feel; it simply is. I can choose if I want to react accordingly or if I try to stuff that feelings away. I tried to do that in the case of love for some years. Did not work well and made me suffer. If I want to see a choice here, it would be the one between doing the "relationship work" or being unhappy.
I can say that poly kind of chose me back then or, put another way, that my feelings finally showed. After I accepted my feelings and stopped restricting myself, all came naturally. I can say, that I suffered a lot more before I embarged on the poly journey. I love my life right now and if I would happen to be single again, I do not think that I would actively seek more than one partner again, but as before, if it happens it happens. I have never actively sought out relationships. They just come into your life with the people you meet.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.