system
Given the following post, respond as if you're commenting on Reddit:
human
What is a birth mark and why do so many people have them?
gpt
Some are blood vessels near the skin (like a capillary hemangioma that grows for several months and then starts to recede). 

Some are just moles, but it makes people feel better to call them birthmarks. 

Some are basically large freckles that don't mean anything unless you have several of them over a certain size (for example in neurofibromatosis). 

Some, like the "Mongolian spot" near the butt areas of some babies, show up in a ton of babies and slowly fade with age like the "stork bite" redness behind the neck or the "angel's kiss" between the eyes/forehead. 

Why do people have all of these benign marks? In the absence of a more sinister condition... just plain ol' unexplained luck or genetics (Mongolian spots more in Asians for example).