I've wondered this myself. I'm a music geek, I spend a good chunk of my time collecting and listening to CDs, buying music magazines and other related junk, and chatting with fellow fans on several Internet forums catering to my tastes.
My wife, on the other hand, doesn't have much interest in it at all. I own almost 1000 CDs, she owns maybe 100, tops, and even those don't get listened to very much. As a matter of fact, my brother bought her a new Carrie Underwood CD for Christmas in 2010 and it's sitting here on the desk, still in the shrink wrap, as I type this.
She says I'm "music geeky enough for the both of us."