system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 - Why do we have moles on our skin?

And why do some people have more than others?
gpt
1. Natural carcinogens in the environment (e.g. UV from the sun) cause mutations in your skin cells. 
2. Some of these mutations cause those cells to grow unnaturally crazy (think first steps of cancer). 
3. In built defense mechanisms steps in and detects abnormal growth or messed up genome that cannot be easily fixed. 
4. Cell decides to self-destruct. 
5. Results in a mole. 

This is a good thing. Following the logic, tendency to form moles means you're less susceptible to melanoma.