Excessive Sweating (Hyperhidrosis) may be caused following reasons.

Excessive Sweating Cause # 1: Genetics

Hyperhidrosis Genetic Transmission

Primary or focal hyperhidrosis usually starts during adolescence or even before, and seems to be inherited as an dominant genetic trait.

The process of passing on hyperhidrosis is believed to be autosomal dominant mode (a type of genetic inheritance) – essentially meaning that there is about 1 in 2 chance of a parent having the hyperhidrosis gene passing it to their kids (if only one of the parent has the affected gene)

Not everyone who has the gene shows signs of Hyperhidrosis. It is believed that about 5% of the population carries the hyperhidrosis genes, though only 3% of them show the symptoms. However, a person who apparently does not suffer from hyperhidrosis may in fact pass along the condition to their offspring. This may provide an explanation for those individuals having hyperhidrosis that do not seem to have any history of the condition in their immediate family.

Excessive Sweating Cause # 2: Underlying Medical Condition

Hyperhidrosis may also be cause by an underlying medical condition including disorder of the thyroid or pituitary glands, diabetes mellitus, tumors, gout, menopause, certain drugs, or mercury poisoning etc, The full list of underlying causes is still not known. This type of hyperhidrosis is called Secondary hyperhidrosis and can start at any point in life, usually in adulthood.

Excessive Sweating Cause # 3: Sympathetic Over Activity and Other Triggers

Some also claim that Hyperhidrosis may be caused by sympathetic over-activity. Nervousness or excitement can exacerbate the situation for many sufferers. Other factors can play a role; certain foods and drinks, nicotine, caffeine, and smells can trigger a response.

According to various estimates, about 3% of  the world's population is affected with some type of Hyperhidrosis.