I'm certain that this one is perplexing to anyone that knows me. For the most part, it confuses me as well. However, I have been able to distill two different reasons for which I decided to do the whole marriage thing.
1) I found someone who loves me and, as importantly, has the same major goals in marriage and in life that I do.
2) Interestingly enough, every guy I know who is married seems to be fairly positive about the whole experience. This could be for several reasons, the ones that I can think of are listed below.
-Cognitive Dissonance. They got suckered into it, so they believe that they must be happy.
-It really is that good.
-It's some kind of conspiracy and all my married friends and acquaintances are in on it.
For the sake of being pessimistic, lets assume that reasons 1 and 3 are each twice as likely as option 2. Thus, a 40% chance of cognitive dissonance, a 20% chance of marriage being all it's cracked up to be, and a 40% chance that there's a Majestic 12/Bilderberg Group/fnord style conspiracy to get me hitched.
It could be argued that if one's brain believes that they are happy, then they are happy, thus it could be possible to define either a 20% or 60% chance of happy outcome depending upon your philosophical definition of 'happy'. (Once again, with fairly pessimistic estimates.)
Am I sure? Hell no. But if I waited until I was sure about everything, I'd never do anything. Besides, even a 20% chance of happiness is better than most anything I can think of that doesn't involve a good beer, a medium rare steak, or a back rub.
Really, the only things that get away with that are the Kobe style beef cattle, and they are rather more committed to the steak part than I would care to be.
