The distinction between "very ticklish" and "too ticklish" is interesting because I think of myself as exceptionally ticklish, like top-of-the-scale ticklish -- apply a fingertip and I'm writhing and barking with laughter. People who tickle me for the first time always feel compelled to add commentary to my reactions: "Whoooa." "Wow, you're really bad." And when I'm being tickled for prolonged periods I'm desperate for it to stop -- I'll promise anything to make it stop. And I'll even say things, involuntarily, like "Stop, I'm too ticklish" or "Don't, you know how ticklish I am."
That said, it's not akin to pain for me, the way some acutely ticklish people describe it. It's reflexive and uncontrollable and intolerable in its own way, but my laughter never shades into screaming, and when it's over I'm not traumatized or teary or angry, as I understand some people can be. It's an oscillation -- I'm unusually sensitive to tickling, but my responses to the stimuli don't go beyond the entertaining spectacle of helpless laughter and comical struggling.
This may be why I've had the experience of encountering a variety of people in my life who've tickled me repeatedly; the fact that I'm not broadcasting abject suffering means their compassion is never triggered; they feel comfortable being a little merciless with someone who doesn't appear to be made miserable by their mischievous attentions. The same way they might prank someone who's a good sport about being publicly embarrassed, but not someone who'll burst into tears. I had a coworker once who tickled me semi-relentlessly in the office and said that her boyfriend was ticklish but the first time she tried tickling him he pinned her down and yelled at her not to do that; it's clear that she found my involuntary reactions to her fluttering fingertips to be surprisingly refreshing.