In fact the feeling of tickle is due to the speed of the nervous signal which arrives to your brain.
So in zones with few nerves, which are not very sensitive .The nervous signal takes very a long time to arrive what causes sometimes a pleasant feeling sometimes unpleasant .
But if you tickles or even masses yourself, your orgnanism is aware in advance of the place where you will tickle yourself and will thus not be surprised, the nerves will be ready and the nervous signal will arrive very fast to your brain what will cause only a banal feeling of touching.
One could find it stupid on behalf of the organism to be developed such as to prevent humans from getting feelings alone but nature is well made and so if such feelings exist it's to support the human contact.