As I've gotten older I have days where I'm definitely in the mood for sex but due to aging and things changing I may only get a little wet, not enough to prevent friction. It's not that I don't want to or I'm not in the mood, I really, really am, just my body has a different chemistry as I age. Pregnancy is another one, your body does some weeeeeeird stuff when you're pregnant and does not cooperate with you at all, I'll be in the mood without a doubt but again my body is not on the same page. And then there are medical conditions where your body doesn't want to produce no matter how many signals it gets that it should be producing some lubrication down there.