BlueCreek

Yes, there are dozens of possible reasons, but one of the things I've learned in the past year is that it is important, really important, to understand that some people are just different with their intimacy needs and there are plenty of people who love their spouses more than they can say, but still not have any interest in sex. It is very hard for the one who places higher priority on it to not take it personally, to not feel lonely or rejected, but you have to try. Often someone who is not a sexual person by nature can be highly so when they first date someone because of higher levels of chemicals the body creates when a relationship is new. But as time goes on, the body stops producing those chemicals and your spouse loses that desire or need they had in the beginning, while you are left feeling jilted and wondering what happened to that great love life you used to have. Then, as Chopblock said, you start begging, pleading, arguing about it, and placing additional stress or pressure on her, maybe even checking out and withdrawing. All those things only cause a negative feed back and soon you're in a catch-22 spiraling out of control.

But hope is not lost and there are plenty of things you can do as a couple to shift things back towards a compromise that satisfies you both. The first step is really talk it out with her in a non-judgmental or accusing manner and try and discover what exactly it is that might be going on. Some of the effort will need to be on her, no doubt, but again, as Chopblock said, you can't force her to do anything, so a big part of it is also how you handle this shift and that you focus your efforts in a positive manner that makes every other aspect of your life with her as easy and enjoyable as possible.

I sent you a message with the name of the book that REALLY helped my wife and I with a situation where there was no real reason for the disparity in our needs other than she just felt no desire even though she couldn't have loved me more.