Why do people marry?

As G reminded me just the other day, I have on occasion claimed that '...marriage is for mad people'. So obviously, this has been a question I have asked enough people at enough times...
I found a somewhat-decent answer in the most unexpected place...

"Because we need a witness to our lives.
There's a billion people on the planet... I mean, what does any one life really mean?
But in a marriage, you're promising to care about everything.
The good things, the bad things, the terrible things, the mundane things... all of it, all of the time, every day.
You're saying 'Your life will not go unnoticed, because I will notice it. Your life will not go un-witnessed, because I will be your witness'."

-- Beverly (Susan Sarandon's character) in Shall We Dance?

Am I convinced?