It's the very condition of being ticklish that embarrasses me; when people discover it, their reactions (surprise, delight, boisterous remarks like "Oh my gosh he's TICKLISH") tend to emphasize the degree to which ticklishness is not a universal condition, that not everyone when touched in a certain way is compelled to twitch, curl up, giggle like a schoolgirl. This highlighting of my ticklishness as a difference, something that sets me apart from "normal," non-ticklish people, is what embarrasses me the most.
Or the highlighting of the degree of my ticklishness as the exceptional quality--most people are ticklish, is the message they're sending, but not as ticklish as you. "My gosh you are REALLY ticklish," that sort of thing. Very embarrassing.
Maybe worst of all is the implication that the quality and degree of my ticklishness is particularly unusual for a male. Everyone expects women to be ticklish but a guy who's so ticklish apparently needs to be dwelled upon, discussed, probed and explored at intolerable length.
The fact that being so ticklish is not just (allegedly) unusual but also incapacitating just makes it worse. That this physical response to stimuli is an amusing weakness that gives the other person temporary and playful power over you just sort of seals the inherent humiliation of it all.