My take
I recently did an article for a D/S E-zine on the topic of tickling. These are only my opinions, but a fair amount of research and discussion with other ticklephiles went into it.
First off is the Dominance angle. As ticklers, we are forcing (wheather consentual or not) a reaction that the 'lee would not be giving if we weren't making them do it. It is up to us where and how they are tickled. Most of us with a Dominant bent can't resist the begging, either.
Second, tickling is perfect for those of us who love to Dominate, but are not into pain. I feel that it is a more personal form of torment, as the laughter triggers a subconscious reaction that it's "fun" rather than painful, saving us any buried feelings of guilt.
Third, tickling is much more sexual than, say, spanking or whipping, as the amount of touching you need to do over intimate parts of the body is greater.
Fourth, and this goes with the Dominance part, is that with tickling you can reduce a tough or determined Submissive to a giggling child-like wreck. No matter how seriously someone may deny that a whipping hurts, there's no hiding the involuntary laughter. Which brings us to...
Five. I have had innumerable arguements with the heavier S&M community over tickling being torture. Many, more than you'd think, will allow you to drip hot wax on them, whip them, even electrocute them...but draw a hard limit at tickling. I think that it's this...the strike of a whip or sting of a spank lasts a second, even though it may sting a bit after. Tickling does not diminish as long as it's kept up.
Lastly, I think that some of us simply have something in the back of the brain that enjoys the visual imagery of a bound female at our control, squirming. You go to see a stripper or get a lap dance to see the dancer's body undulating and wiggling, showing off the parts that you like. Same with tickling. I for one find very few things as erotic and sexy as a helplessly wriggling female body.
Above all is the relationship issue. There is no damage with tickling, and lovers can get very close to one another with tickling. The fun of exploring each other's bodies in search of the "T-Spot" is something I have had the pleasure of enjoying with many partners and it cannot be described in its sensuality and closeness.
Well, that's my take, hope it was some help.