Feet have a lot of nerves in them. But if you tickle someone somewhere enough these nerves get less sensitive. Therefore someone who is tickled by having their feet touch must have very sensitive feet, but those who enjoy it have less sensitive feet.
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The induction of laughter by tickling is a reflex, making it closely related to the startle reflex. It is believed to have evolved in humans over time as a protective reflex, as the areas in which we are ticklish are also the most vulnerable. This shows the ties between we and our other mammal cousins very closely; like nearly all mammal families who exhibit play-fighting in the wild, human parents (just like other primates, mind you) tickle their children in order to encourage the development of combat and defensive skills. The laughter that is associated with tickling is because positive reactions encourage the tickler to continue, whereas a negative reaction would discourage them and thus eventually lead to a more physically vulnerable animal.
Some people are tickleish because they & some people are not because they not and some people just dont like to be touched by other people