Levels of sexual desire vary among different women. The same woman may vary widely in how sexual she feels, depending on her health, how much stress she's under, where she's at in her menstrual cycle, etc. Alcohol, some medications, and drug abuse can mess up a woman's sex drive. Subtle hormone changes as one approaches menopause also have a large effect. Sexual interest, arousal, and performance are all dependent on a complex interplay of brain neurotransmitters as well as both the balance and levels of male and female hormones.
Diseases affecting the nervous system can directly repress sexual desire.
Debilitating chronic illnesses can use up energy that would normally go towards sex. Thyroid disorders or endocrine problems can disrupt one's sex life including increasing sex drive in women with excess male hormone production or testosterone use. Sexual disinterest is also a major symptom of neurotransmitter problems such as depression. Emotional stress is a common cause of lack of desire, especially relationship conflicts. Once a difference in sex drive has turned into an issue in a relationship, it can take on a destructive life of its own.