There is a recent thread that touches on whether poly is a CHOICE or an IDENTITY for people...
Here is the link (hopefully): Is polyamory a sexual identity, just like being queer?
To some people, it is just a relationship structure that makes sense. To others it is some inherent quality. Some people identify as poly (capable of loving more than one at one particular time), but live monogamously. Almost every person on this forum has a different story. Some may share common themes, but people turn to or discover polyamory for a variety of reasons.
Personally, I discovered the term polyamory and it just fit what I'd been feeling: the desire to continue seeking (or following up on potential) relationships and what could have become romantic love for a close friend while still being 100% in love with my husband. I didn't join a society, and I've actually never met anyone who identified as poly BEFORE having me explain the concept to them.