As a 'ler it's being able to make your partner's body writhe and squirm with a touch. Watching their body move whilst your hands feel, and the sense of control over their thoughts and motions. If they are enjoying it too, all the better - seeing someone you find sexy getting turned on by your actions just has to be the hottest thing in the world! Also, (speaking as a male) girls look so amazingly pretty when smiling or laughing. Being able to cause that at your own will is a huge turn-on. Even kissing a smile is so hot! Sure, it may not be the most technically accomplished kiss if she's smiling or giggling, but the feel of your lips on hers is just amazing at that moment.
As a 'lee it's the mixture of pleasure and panic, over which you have no control. Your partner can dominate your thoughts and motions entirely with just their fingertips. The handing over of control to them releases any inhibitions, it allows you to 'be sexual' in it's purest form without any sense of responsibilty over your own reactions. A form of non-painful torture where their touch can make you react according to their will, and not your own. You become their 'plaything'. Just the feel of their hands over your body is hot enough in itself.
For me, tickling is a kind of BDSM-lite! It has all the goodies of the power-play and domination, only without all the nasties (such as pain). A more playful form perhaps?
Also, it's a fetish of course. There are plenty of those out there that seem to defy explanation. What causes it? A 'misfiring' of neurons perhaps? An unconscious memory of sexual association? There's plenty of theories of fetishism, but nothing that I could yet be satisfied as definitive. And I mean "yet" - the whole concept interests me so I would love to see more posts in this thread.
It wasn't always the case, but I'm actually pretty thankful I like tickling. Compared to some of the alternatives it's pretty inoffensive to any 'non-believers'. It's something a little different to the 'norm'. And (if I can be excused for putting it so bluntly), who wants to fuck average?