I'm thinking that she's Peri-Menopausal given her age.
But, if she got a clean bill of heath and there's no hormonal imbalance;
then, there could be a number of reasons... including Depression.
You would have to ask her if there is something wrong with you. It sounds like she loves you but for some reason is not in love with you or has some pent up resentment toward you, or just does not find you sexually attractive anymore? Im guessing here.
Well you sound like the perfect husband to me, could you talk to mine about doing housework? lol
But onto more serious matters. You should sit down and have a talk to her about how your feeling,and I mean you HAVE to tell her how you feel and be straightforward no scooting around the issue. The only way you can really find out what's going on is by talking to your wife about it.
In all honesty she very well could be bored. NOT necessarily with you but maybe with the sex life. Do you try different things or is the sex always basically the same? Sometimes you can get into a pattern, where the sex is always the same basic thing and you need to get out of that and spice things up a bit.
Some days I don't want to have sex with the hubby and it's mostly because he'll come home from work and not really give me any attention until the point where he asks me for sex. If he's ignoring me the entire day i'm not about to give him sex if you catch my drift. Do you talk to her, ask her about her day and just cuddle occasionally? Those kind of things can really change my mind about it. If the hubby comes home and just cuddles(and i mean JUST cuddle no probing for body parts lol) with me on the couch it gets me in the mood. If you're just asking her for sex all the time and not doing anything else with her, She's not going to feel loved and she's going to think that sex is ALL you want from her.
I hope this helps you out, and please just be completely honest with your wife when you talk to her. :)