1. Use of oral contraceptives. Some women will find that their sexual drive is decreased by the Pill (or any of the other hormonal approaches to birth control -- patch, ring, and shot).
2. Use of antidepressants. Not everyone experiences a decrease in sexual desire, but many do.
3. Breastfeeding. Prolactin (the hormone that facilitates breastfeeding) decreases sexual interest.
4. Lack of sleep. For most women, sleep comes before sex once the relationship has been established.
5. Stress. Due to work, financial issues, educational stress, extended family, and other important issues in life. When stress is increased, many women do not see sex as a solution to it.
6. Disagreements with one's mate. Any relationship will bring with it the challenges of conflicting feelings and desires. That can play itself out in sex.
7. Low levels of free testosterone. While knowing the level of total testosterone in the bloodstream can be helpful, finding out the free testosterone is very essential to discovering the possible physiological causes of low sexual desire.
8. High levels of SHBG. A woman with high levels of sex hormone binding globulin (SHBG), may have low sexual interest. This is because it combines with free testosterone (making it "unavailable") and that decreases libido.
9. Fear of intimacy. The inability to handle the level of intimacy that sex brings and maintains in a relationship is a very common reason for a decrease in sexual desire for one's partner.
10. Body image. Women who view themselves as unattractive to their mate and/or in their own mind's eye.

via: http://top-10-reasons.blogspot.com