While there are numerous risk factors, skin cancer is usually caused by UV radiation from the sun. Excessive exposure to this radiation is the most common cause of all forms of skin cancer.
The two principal types of UV radiation you are exposed to penetrate the skin to different depths. Too much time in the sun, and the rays begin to damage DNA and tissue. At best, this causes premature ageing and wrinkles. At worst, this damage leads to permanently weakened skin and, eventually, skin cancer.
Limited exposure to the sun is good for us. UV-B rays stimulate the production of vitamin D3, which is important for good health, growth and strong bones. However, a small dose of sunshine each day is all that is needed.
While sunshine makes us feel good (hence the popularity of sunbathing), the damage it does to the skin and body far outweighs the short-term benefits of being in a good mood.
The damage the sun does to the skin is permanent. While sunburn may fade, the underlying changes to the skin remain. Each subsequent period of prolonged exposure adds more damage to the already-weakened local immune system and genetic material of the skin.