Some people tell it like you just finally find "the one." And some people say it's just work, you pick someone and they pick you and with a little work and a little luck, it works out. Some people say, "I wasn't ready till now, I had to grow," and some people say, "I was waiting for you my whole life." Romantic love contains within it the idea of knowing, and thus, for some of us, requires a leap of faith.
I think you change, and keep changing, all the time. And if you find "the one", it must be that you've changed in some way that has made that possible. You've found someone with whom that particular part of you emerges, the part of you that can imagine one person, forever.