Marriage

If you're looking for a reason, here's a decent one:

But wait, there’s more. Marriage doesn’t just cut expenses. It raises couples’ income. In the NLSY, married men earn about 40% more than comparable single men; married women earn about 10% less than comparable single women. From a couples’ point of view, that’s a big net bonus. And much of this bonus seems to be causal.

It makes you richer.