I don't have kids (and never will) but I have this as well. I'll be driving and suddenly start worrying that I'll fall asleep, drift into the other lane and plough into a truck. We built a new fence at the back of our house the other day. Couple of hours later, I suddenly have the thought "Good God! What if somebody sneaks into the house with the nail gun and nails my wife in the head in her sleep!". My wife says she has the same sort of thing. Maybe it's something that happens when you're comfortable in life. Maybe it's your brain coming up with ways it could all be ruined or something?