You're with a significant other. You like each other, very much. So you decide to get married. But how can you ever possibly know that you are with the best possible person for you? There is no way you can sample everyone in the world, and there are loads of people that can outdo your other in any category. Asserting that they have the perfect combination of characteristics for you seems a bit silly, as you don't really know that -- you don't know what specifications the perfect person for you has, or even what all the specifications of your partner are, not all of them. Besides, perfection is something to be striven for and never achieved.
If you concede that you can never have the perfect partner, or if you do, you could not know it,
I don't mean to imply that if you settle for something great, realizing that you will probably never have perfection, you could find your soul-mate, though it's theoretically possible. The point here is that if you can never have perfection, there will always be something better*.
Knowing this, how can you ever make such a commitment as marriage, when any day you know you could meet that someone? Then, when you meet someone you think is better for you, you have to make a decision: either you break your vows of commitment and take a shot with this new person, or you suck it up and stick with your current person, whom you like a lot, but now you think that there's a person that is even better for you, and now you will never get the chance to be with him/her.
Doubt and uncertainty plague the lands and all you can think about is that other person and what you might have had together were it not for all these little snot-nosed goblins doing their best to destroy all the things in the house that can be, into sharp shards, to then fall on and hurt themselves, or you, you could be even happier, which is actually easier to accomplish now that your world is plagued with the aforementioned doubt.
I guess that was the reason behind marriage in the first place: creating a stable and positive environment for kids to grow up in, not having them worry about the guy that beats up dinosaurs and drags them back to the cave choosing a new cave and not informing his kin [unsure about proper punctuation here]. But I don't think that's why people get married now, at least not for the most part.
So how can we take those vows today, how can we promise to be with another person until death does part us, knowing that you could run into someone that would make you either a liar or misearble until death?
thread rule: you aren't allowed to use the word "love" or you lose. Except me. I am allowed.
* While the mathematical relation of people's worth (one person being better than another) might seem overly simplistic, I think it gets the point across.