Excessive dryness can be a very annoying issue that comes into almost every woman's life at one point and is normally caused by a lack of foreplay or age.
Physical Factor
Physical factors that cause dryness can be cause by several issues such as; Damage to the spinal cord, A side-effect of medication or narcotics, low hormone levels, and an issue with your glans.
If something is Physically causing you to be dry down there, you may need to get some form of treatment, or perhaps change your prescription.
Psychological Factor
On the other hand a women may be dry simply because she isn't mentally or emotionally "in the mood", this can be fixed easy enough with foreplay and romance. Remember if you don’t feel like having sex but get into it for your boyfriend or lover, then you will probably be a little dry.
Some women who suffer from a psychological impediment are more inclined to not want to have sexual intercourse altogether than just being unable to naturally lubricate themselves. Some reasons for a psychological inhibition are: Traumatic sexual experiences, Excessive stress, Relationship worries, Anxiety, Depression and psychiatric conditions.
