Tickled pink, and then green

Why You Can't Tickle Yourself, article.
... But, when the robot introduces a delay of one-third of a second between their left hands moving the stick, and feeling the sponge move on their right hands, suddenly it tickles!

All well and cool, but it doesn't explain:

• What *is* "being tickled", the sensation? 
• What is the purpose of it? 
• Why is the reaction laughter? 
• Why is it so unpleasant despite the laughter? 
• Why will so many people torture kids with tickling, despite that they know it's unpleasant? 

... Maybe it's only really unpleasant to Highly Sensitive People?