For me, it's innate. Or at least, non-monogamy is. I have always struggled with monogamy, made it clear to my husband for years that I would prefer a non-monogamous relationship, and ultimately reached a breaking point where I don't think our marriage would have survived if I had tried to remain monogamous any longer. I didn't know for certain that another actual relationship was what I wanted until I found myself in that position, so I didn't necessarily know that poly specifically was right for me, but I knew that monogamy was wrong. I tried to repress that for years, and it was never going to work.
Not everyone feels that non-monogamy is that firmly embedded in who they are, but for me, it's never felt like a choice. My opinion is that it's not necessarily something a person should try to "overcome" for the sake of someone else. But that's only my perspective, having spent years trying to do just that.