If your spouse has a lower sex drive than you, that doesn't necessarily mean that there is something wrong with them. You may just have a higher sex drive than they do and this is not uncommon or abnormal according to Dr. David Schnarch (author of Passionate Marriage)
If you are the one with the lower sex drive and it troubles you, you may want to take the following into consideration.
This slideshow from MedicineNet.com shows some possible causes of low libido or lack of interest in sex. Most female or male sexual dysfunction can be easily remedied as this slideshow will demonstrate.
Sex Drive Killers - By MedicineNet.Com