Answers:
When someone else tickles you and you counter it is a form of your "brawl or flight" response. Your body is react to percieved peril. Your body know that you, yourself, are not placing it in risk and as a result does not turn on this response. So, you can not tickle yourself.
because youv get no fingers
Mainly because in the exact time you're doing it, you already have thought of it and your (mind) fretful system have made provision for that!