Birthmarks are one of those mysterious things that science hasn't quite figured out yet.
While it's known that most are the cause of overgrowth of blood vessels or certain kinds of skin cells, the actual underlying cause is still unknown.
Even though a large percentage of babies have or develop a birthmark within their first year, people with large or obvious ones still feel judged or ugly.
That's why it's so great when people choose to share the beauty of their birthmarks instead.