Drugs, alcohol, and nervousness all play their part.
Everyone is familiar with the central nervous system that is our conscious control over our bodies- movement etc. There is another nervous system made up of 2 parts called the sympathetic and parasympathetic nervous systems.
These systems work to control all other bodily functions that you are not aware of such as digestion, heart beats, breathing, temperature regulation, hunger, sleep etc. Basically everything that keeps you alive.
The 2 systems act to balance each other out. For instance when you eat, your parasympathetic system takes over and looks after digestion of food, releasing enzymes, insulin etc to control body sugar, fat / cholesterol absorption etc.
The sympathetic system works when your body is more stressed,leading to an increase in blood pressure heart rate etc amongst its many other functions. Overactivity of this system causes your body to produce more sweat. When the problem becomes excessive one method of treatment is to divide the nerve fibres in the sympathetic system to reduce the sweating.