It's probably just an issue with oxytocin.
It does a lot of stuff, but what's important here is it's about social bonding. When it's not functioning correctly, you just never get lonely, you don't have the negatives from lack of social relationships including romantic.
Also, it's what make orgasms feel amazing. So without it, your getting rid of the two main positives for sexual relationships.
You might still enjoy the act of sex, you just don't get the huge addicting high at the end of it.
So it's at most a "sure, why not" thing compared to everyone else. And for some people, it's just not worth the hassle to varying degrees.
We also haven't identified any genetic markers, but there's likely some out there. And there's environmental issues when young that can effect it your whole life.
So we barely understand the "nurture" component and no idea about the "nature" side of things.
Edit:
Rather than keep responding to the same comments, please just scroll down for more information before replying.