I think it's very much down to temperament. The people I know who believe in "The One" and "just knowing" and all that, tend to be more impulsive, enthusiastic, all-or-nothing types who have little trouble making decisions and throw themselves into things. When they meet and fall in love with another similarly minded person, it's intense and exhilerating and feels like The One.
I, on the other hand, analyze everything, am cautious and take a long time to make decisions. I am not prone to extremes and have never "just known" about anything, be it a job, a decision, a feeling or whatever, so it stands to reason I'd be the same with relationships.
So while I may feel very well suited to my partner and happy with him, it isn't in my nature to believe in The One.
Some might say it's because he isn't The One, and I would know if he was etc etc.
But he's my first love, we've been together for eleven years and seen many "The Ones" come and go for other people.
So I tend to think it's down to personality and compatability.
Having said that, I believe in "soul mates" - and not just in a romantic sense- in the sense that there are several people in my life who speak to my soul and are kindred spirits.