I suspect many people don't so much "become" poly as discover that they've always had that inclination but didn't have a word or concept to describe it.
Growing up, I always knew that monogamous marriage was something I would never want. It sounded dreadful, the mere thought of signing over my ability to have relationships with people I liked. I was aware of this long before puberty, long before I was dating, long before I had any sense of romance. I just believed marriage was worse than a death sentence.
Eventually I discovered the formal concept of polyamory and realized that's what I'd felt all along. So from that point on, whenever I was single, I still identified as poly. What it meant was that I had no interest in entering exclusive relationships.
Poly is a description of my tendencies, not my current relationship styles. At many times, I've been in relationships where I wasn't interested in anyone else. I've always been choosy and never had a strong need to be in romantic relationships at all.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker