Like Redpepper, I feel that I am polyamorous by orientation instead of choice or dictated by relationship dynamics or number. I tried monogamy, failed spectacularly at it, and finally came to accept that it didn't fit my life. I could never quite believe the one true soul mate, or other half concept. The thought of only one partner for all of my adult life made me claustrophopic and antsy.
I do not feel that everyone is capable of being polyamorous or being in a polyamorous relationship. I believe some people are simply not capable of loving more than one person anymore than they can choose their sexual orientation.
Yes many of our choices are dictated by societal pressures, but this does not mean, even without this, that everyone would be capable of every relationship type.