Because once your brain matures and you have faced your own mortality, some people realize its not a matter of compromising or sheepishly following tribal behaviour. It actually makes sense to some. For me, it's a way to facilitate spending the limited amount of time I get on this planet: by being with my family.

Marriage is a nice, acceptable institution to facilitate family life. Upsides are the ring, the party, taxes, no legal bs when having kids and the ease of buying a house. I've yet to find a downside... Prerequisites are the conviction that your SO is the one you want to be with for a long time, without exception.