Sex drives are an interesting thing. Hormones and physiological stuff are involved, and mental and emotional stress can definitely have an impact. Back in the day, I was selling sex toys at parties for women, and I always got at least one woman who couldn't get her husband to sleep with her. I used to cruise sexual health boards and see what people were posting, and there were a crapload (technical term) of women posting that their men wouldn't have sex with them, and they didn't understand it.
What people sometimes fail to realize is that you can have physical issues, emotional issues, or relationship issues that effect your sex drive. You can be on medications that affect your sex drive (antidepressants, anyone?) If you have one of those issues, you have to figure out what it is, but many people just avoid this. They don't want to think about it, or really take a look at what's going on enough to fix the problem.
We are a nation of distraction. If we are unhappy, we'll distract ourselves with video games, the internet, TV, food, drugs, alcohol, etc. All of this affects our sexual drives and feelings and relationships.
And then there's just the issue of people having different sex drives. I think this is where polyamory can be a great thing. It leaves the opportunity for someone with a higher sex drive to get what they need, without leaving their partner, not that polyamory is all about sex.