Ok, here's my two cents... I became open to the idea of poly because I have a multi-faceted personality to an extent that makes it unlikely (if even possible) for one person to fulfill all of those facets.
To elaborate, I enjoy moose hunting, camping and fishing, yet I feel just as comfortable in an evening gown & heels at a concert. I can spend hours talking to someone about dogs and search & rescue training, or I can spend hours talking about the theories of quantum physics. I can operate power tools and snowmobiles and I can also make crafts and create works of art. I adore "cowboys" and feel totally at home on horseback... I also speak fluent German and passable Spanish and feel equally comfortable in a completely different country.
So... for me it's not that one person isn't "enough" per se... it's about the fact that each person has something different to offer and it's not fair (or logical) to expect one person to be "everything" to someone.
"Life is too short to always sit around worrying about the bad shit that could happen. It's a lot more fun to go start some shit of my own. "