I believe in the concept of kindred spirits.
It happens frighteningly infrequently, but it happens. I think I have come across only a handful of these people in my life. You can recognize them by how much you have in common, in spite of completely different backgrounds. And then there is the much more elusive creature: the person you have next to nothing in common with, and yet you have a certain understanding. A quiet calm about the relationship in which there is no work, no awkward silences [although there can be silences, if that's your thing], no confusion. In spite of divergent views on religion, politics, music or lifestyle choices, there can be a simple link that remains present below the surface.
I once met a girl in a bathroom who wanted to study Russian. We had no classes together, it was pure coincidence that we ended up living in the same dorm. Through our collective adoration of one particular region, we also found that we have a similar distaste for large social gatherings, love of books and heated debate over coffee and one too many cigarettes. Whereas we came from polar opposite backgrounds, we found ourselves on the other side with many comparable personality traits. This does not happen every day.
I once met a boy in a masonic lodge who wanted to grow up to be an actuary. We did not go to the same school, we had no mutual friends [in fact, he didn't like mine and I didn't like his]. He refused to drink or smoke or stay out past curfew, and I, well, I think you could probably guess. What did we have in common? Nothing. And yet he became a large influence on who I am now for a reason I still don't understand, other than this particular sort of mutual understanding that goes beyond logic. This, also, does not happen every day.
I don't think that kindred spirits are drawn to each other. I believe that you stumble upon them in the same manner that you can stumble upon anything in life, purely accidentally. Once you meet, however, it is difficult to shake the connection.
"There's no accounting for chemistry."
"We're on the same wavelength."
"Some people should only play a bit part in the story of your life, some have major roles."
"I could set him free and he'll only come back."
Whatever cliche you want to use, I am grateful for my kindred spirits, as few and far between as they are. They remind me of a lesson I've learned repeatedly in the past few years that you can have everything in common and not get along, you can have absolutely nothing in common and remain close. You won't know until you meet someone, but at least in my experience, it is simple to tell right off the bat. You can work at a relationship your entire life, but with a kindred spirit you won't have to.