something weird about me

I rode my 4 wheeler this morning with some friends up a mountain trail. When we got to the top, some folks started leaning over the ledge looking down. They invited me to join them and I declined. I simply can't do it. I've encountered this situation dozens of times in my life. They ask if I was afraid and I told them no, they poked fun and heckled. Then I told them why I can't do it. Whenever I get to a ledge that isn't roped, fenced, or walled off, I get an uncontrollable urge to jump. In my youth I've had to be restrained. I seriously can't control it. The only way I can is by not looking over the edge. I can get as close as 3-4 feet depending on the slope. Sadly, I think I have passed this trait along to my son. On playgrounds, if he ever finds an opening that is unobstructed I have to stand there because he jumps. I don't know exactly how high the ledge has to be for me to feel the urge. I know 30-50 ft isn't a problem for me and I don't feel compelled to jump. But I know 100 starts the urge. The higher the ledge the greater the urge. It gets weirder. I love rock climbing. I can ascend a cliff look down and have absolutely zero compulsion to jump. I have no compulsion while on a rope climbing. I don't get the urge on building tops or airplanes. I do on dams, I think the urge might even be magnified when I see water.