Why can't you tickle yourself?

I’ll tell you why this question occurred to me, I’m an ICU Nurse and I was evaluating an anoxic brain injury patient and looking at her Babinski reflexes. You swipe the bottom of the foot and the toes curl. But here’s the thing, if I do that to me, it doesn’t work. Knowing that brought me to the question. I mean, I can’t tickle myself either, but I can be tickled, and my motor reflexes (like the Babinski) will work if somebody else provides the stimulus. Why is that? Why is the outside stimulant the factor here?