The most fascinating thing I’ve read in a long time: Why We Can’t Tickle Ourselves, from Mind Hacks:
Tips & Tools for Using your Brain in the World.
As I understand it, apparently there is a way by which our body distinguishes between stimuli that is self-inflicted or externally inflicted: whenever we act, our brains predict the effect of the action…and that prediciton is then compared to incoming sensations. If the prediction matches up with the sensation, then we know that the sensation was caused by us, and not by something external. (The way they explained it was a bit more complicated, but this seems to be the general idea).
We feel external stimuli more intensely because it is external stimuli which is more important to attend to — what we are doing to ourselves is really not as (evolutionarily?) important as what something (or someone) else may be doing to us. Hence…when we are tickled by others it feels more “tickly” then when we tickle ourselves.
What was the most delightful to learn: mice are ticklish.