By far, the best answer that I have ever heard to this question came from the recent movie Shall We Dance?:
"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'."
Today is my 4th wedding anniversary.
My life's witness is an amazing man with seemingly unending patience and an uncanny knack for finding the humor in every situation (well, he tries to at least). He gives me his friendhsip unconditionally, no matter how much of a bitch I may be. He's the yin to my yang; my court jester; my unknowing (yet accepting) target when my hormones get the better of me; my reluctant confidant when I am feeling catty; and most recently, my advocate during a recent time of crisis. He indulges my rants, my cravings for Ben & Jerry's Chubby Hubby ice cream and my occasional superficial need for a little bling-bling now and then. He's a devoted father, hard worker (sometimes to a fault) and a man of intense moral integrity.
There's an old gaelic phrase -- which I had engraved on the inside of his wedding ring -- "anam cara" which translates into "soul friend." This is my husband.