Two main causes of moles are genes and exposure to sunlight. Some people are genetically more likely to develop moles than others and hence, have more moles. Sunlight contains ultraviolet radiation which can cause aging of the skin and skin damage in large amounts. Skin aging and damage may result in moles.
Moles can develop anywhere on the body but usually develop in areas which are often exposed to sunlight e.g. arms, face etc.