A woman’s vagina naturally produces a clear, relatively odorless vaginal lubrication. How much lubricant is produced is influenced by a number of different factors and varies from woman to woman. Factors that may influence vaginal dryness are:
Allergens such as dyes and fragrances in toilet paper and laundry detergent
Drying soaps
Low estrogen & progesterone
Menopause, Perimenopause and postmenopause [Hormone Fluctuation]
extreme stress, fatigue and emotional disturbance
Some of the most common reasons are:

Menopause - Menopause is a natural part of aging. As a woman matures and approaches menopause, her estrogen levels drop, which can cause vaginal dryness. While most women experience menopause naturally, some women may undergo premature menopause caused by a surgical removal of the ovaries, the organs responsible for producing estrogens. Following the surgery, the production of estrogen decreases abruptly, often leading to vaginal dryness.
New mothers - Normal hormonal levels are not always reestablished immediately after delivery. The common cause of "postpartum" vaginal dryness can be the hormonal imbalance that can occur from breast-feeding. In addition, stress to the vagina from the trauma of childbirth, an episiotomy, or from a vaginal tear or laceration, can cause temporary vaginal dryness.
The Pill - Oral contraceptives affect all parts of the body with synthetic hormones. These hormones can cause both physiological and psychological (such as stress and tension) changes that result in vaginal dryness.
Emotional Stress - Everyday stress may also lead to lack of lubrication. Nervous tension and fatigue can contribute to this problem, as well.
Other surgery, infection, and even various medications can lead to vaginal dryness in many cases.
Vaginal dryness can occur after a menstrual period or after tampon use.

Author's Bio: 

Dr. George Grant, Ph.D., I.M.D.
Specialist in Integrative Medicine