You're brutal.
It drives me nuts to see the fingers approaching, especially if they're aimed at an extremely sensitive and helpless spot like my exposed armpit. But I can't look away. I have this deluded sense that if I'm watching I can still make it stop somehow -- say something or do something that will get me out of the situation -- even though watching the fingertips draw closer makes me ever more frantic.
Like I've said a million time before, you'd be so much fun to tickle! If you're wife ever needs some help, I'd be more than happy to volunteer!