Welcome aboard. I am pretty new here myself.
As for why I decided to be poly, I had never heard of it until fairly recently. A friend I made was poly and she turned me on to the idea. I have always been a big proponent of honesty and openness. Through my life I found it easy to form little crushes on people. In a couple cases this blossomed into actual feelings for them. I was usually in a monogamous relationship when this would happen and I would feel ashamed about how I felt. I didn't know at the time that it was just natural and normal. When I heard about polyamory I looked back on those parts of my life and saw I didn't have to be ashamed. It was possible for me to explore these feelings. It just made so much sense and I wanted to make it a part of my life.