Well, in the past, you married a girl, or made her pregnant, and bim, she had to deal with your dirty ass forever and to bake cakes for you. Also, a quote attributed to Socrates says something like "get married, if it's a good wife, you'll be happy, if you get a bad one, you'll be a philosopher". Unfortunately, I'm too young to trap someone into baking me cakes :c
On a more serious note, I think that marriage is, nowadays and probably in most of Europe, the culmination of your love to your partner. Culmination may be a too strong word seeing the divorce rate, but that's the theoritical part.Thus, I think that's a decision you take when you are sure that you love someone and that you want to inscribe that love into societies, a family, et caetera. To give your love a more tangible shape.
Surely getting financial/social advantages isn't out of the equation either.