As a 'ler, I can't answer MRERIC's question, but it is something that I am endlessly trying to understand - the psychology of the 'lee. I am full of awe and admiration for you 'lees, and thank heavens for you and the way you keep coming back for more. I can only surmise it must be similar to the psychology of the roller coaster rider (and I can't do roller coasters either) - an attraction to the sensation of helplessness and exhilaration in an extreme situation where you know that ultimately you are perfectly safe?
But it is weird that this sensation can be uncomfortable and disturbing yet simultaneously so powerfully attractive.
Maybe tickling is disturbing because it is so invasive - it is basically an attack. And yet it can become something that we seek and desire, because it is also a non-violent, very personal and responsive interaction, and as human beings we all want closeness and intimacy. Tickling might be compelling precisely because it combines that powerful element of attack/control/invasion with exquisite mutual responsiveness and intimate contact. Another thing - maybe the only other thing - that can have that combination is sex. Tickling might have its hold over us because our brains and nervous systems find it so weirdly akin to sex?