For the people who don't believe in "the one" is there any particular reason or you just don't believe in it?
Yes because there are literally billions of men on the planet who I'm sure I could love and who might love me back, if only we'd met :)
Most people end up with someone from a pretty narrow pool of 'availables' although online dating has changed that a bit (it wasn't around 35 years ago when I was single.) Work, friends of friends, sport, hobbies etc.
So, if most people consider who they have dated over a period of a few years, and then ended up marrying one of them, the odds are pretty high. For example, say I went out with 20 men between the ages of 18 and 30. Two were long term ( 3-5 years each) and of the rest I could have happily married two others. So that's 4 out of 20, as possible life long partners.
Given the billions of men out there, had I kept looking and not married DH, I think someone else suitable would have come along at some point if our lives collided.
I have a lot of interests and all the men I've been close to have been very different, so it's not a case of one person (type) being a perfect fit.