If you score more toward the introvert end of the introversion and extroversion spectrum, then compared to extroverts, you're more sensitive to (i.e., less tolerant of) an array of sensory stimuli: electric shocks, lemon juice in your mouth, background noise when doing tasks. Eysenck's idea was that introverts have higher coritical arousal levels, or their brains are more "excited" naturally, whereas extroverts' brains are less naturally "excited," compelling them to seek out stimulating environments to boost their excitement level to a more or less universal comfort level. Introverts, by contrast, don't require much more stimulation to reach this comfort level, and thus are fine being by themselves.
I googled around and found literally nothing on introversion / extroversion and degree of ticklishness -- as in, zip either for or against a correlation. I lack university access, so I can't do a more sophisticated search. Does anyone know of any studies bearing on this? If not, personal anecdotes are welcome of course, bearing in mind that the N will be very small on this blog. Me: very introverted, very ticklish, esp in the rib area -- it's like being electrocuted.