Cancer is caused when damage is done to DNA in a cell which causes the mechanisms that stop runaway growth to break. Such damages can be caused by chemicals (e.g in smoking) or by radiation (light etc). The higher the energy of the radiation the more likely it is to cause damage. Visible light is very unlikely to damage cells. But sunlight, as well as visible light, has a lot of ultra-violet light, which is higher energy and is therefore more likely to cause damage to the DNA. The UV doesn't penetrate far into the body, so the cells which it will damage are the skin cells.

Vanity. Pure and simple. They are gambling against the house (nature). I think it is cultural and sometimes they have seen their parents lying in the sun so they disregard the growing body of scientific evidence that - “According to The Skin Cancer Foundation, most ‘people receive about 80 percent of their lifetime sun exposure before the age of 18. Just one blistering sunburn in childhood is estimated to double the risk of melanoma later in life.’ This is because skin cancer can take 20 or more years to develop”. TIPS FOR SAVING YOUR SKIN 1. Limit sun exposure, especially between 10:00 a.m. and 4:00 p.m., the peak hours for harmful ultraviolet (UV) radiation. 2. Examine your skin from head to toe at least once every three months. 3. When outdoors, use a broad-spectrum sunscreen rated SPF 15 or higher. Apply it liberally 30 minutes before sun exposure and every two hours thereafter. (Sunscreen should not be used on children under six months of age.) 4. Teach your children good sun protection habits at an early age, for the damage that leads to adult skin cancers starts in childhood. 5. Wear protective clothing such as long pants, long-sleeved shirts, broad-brimmed hats, and UV-protective sunglasses. From June 8th, 2005 Awake! Magazine

Why do you think women risk osteoarthritis when wearing heels? :P Well, first of all, most people feel that its not going to happen to them. Secondly, most people are ignorant about the health concerns. People who are tan addicts are usually youngsters who are more concerned about their looks than their health. Anyway, I don't think a lil sunbathing with proper protection would cause skin cancer.

Because they're burning their skin off with ultraviolet radiation? Doesn't really need much more explanation does it? hehe They're soaking up radiation that causes Cancer, thus they are at risk of getting Cancer.

UVb light rays present in the sunlight have the wavelength range 315-280 nm. Penetration of these rays in the skin is dangerous and may causes skin cancer. http://www.antivitiligo.com/vitiligo-treatment/uvb.html