ARI - A very interesting question and one that has been considered for litterally millenia. (Plato, Aristotle, Galileo (is there anything he didn't do?) and Darwin.(Take a look at the tickling section in Darwin's "The Expressions of the Emotions in Man and Animal.")
Even after all this time we don't have definitive answers. Until very recently there have been no empirical studies. (I wonder why? Since so much thought was devoted to it, why was it not studied?)
We have some interesting theories as to the purpose of the tickle response.
Tickling helps parents and child bond. When tickled, babies smile, which makes the parent smile. It's a shared pleasurable experience (actually it's more accurate to say that it's a perceived shared pleasurable experience. Many don't enjoy being tickled. Some actually hate it. But the very young will smile anyway.)
I think the biggest problem is explaining the surface expression and vocalization of pleasure (smiling, laughing), with the actual feeling of discomfort or trying to escape exhibited by many people. Why such a contradiction b/w appearance and feeling?
The most intriguing hypotheses I saw presented on PBS, by a psychologist who did an experiment involving a "tickle machine" that enjoyed it's 15 minutes of fame in the public media. She felt that the two expressions were both adaptive and not contradictory as they serve two different purposes. The smiling and giggling serves the purpose of social bonding b/w parent and child. The discomfort serves the purpose of teaching to protect the vulnerable parts of the body.
There are many psychologists who feel that excessive tickling especially of children is a form of sexual abuse. I'm not very familiar with this theory, but it is in vogue now. I can more readily see a link between tickling a person who doesn't enjoy it and sadism.
Yet there are people who do enjoy being tickled, and there are those for whom it is part of sexual play.
Clearly there are more questions than answers.
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