first, thinking is a dangerous game. it ain't always a good idea. 😛
now, as to your question: i've got many friends, many of them who are ticklish. but i've noticed that, in some cases, some of the most ticklish people i knew weren't really the smiley/giggly type. not brooding, mind you...just not outwardly amused. and yet, i've known some not-so ticklish people that would laugh harder than anyone after a good fart joke. i guess it all depends on the individual.