The exact cause isn't completely understood, but there does seem to be a genetic component. If your parents have them, you're more likely to develop them too. Age is another factor - they become more common as we get older, though they can appear at any age. Hormonal changes, pregnancy, certain medications, and even climate have been suggested as potential triggers, though the research is still ongoing. Unfortunately there's no proven way to prevent them from forming. The good news is they're completely benign and purely a cosmetic concern for most people.