I've never tried to break it down to myself before. I love tickling, always have. Sure the laughter is a huge part of it. Everyone loves laughter or making people laugh. When done through tickling, it's a shared event between people. You causing it and she giving it up, even if they don't want to (as in if they are trying to resist). When it does come, it's a rush of adrenaline on their part, and in turn exciting(not necessarily sexually) for the ler. The whole act is a shared event and a bonding activity. The laughter itself is intoxicating, as part of the event as a whole. The power exchange on both sides is similarly addicting. On the ler side, forcing the laughter from someone, taking it from them if you will, and on the lee side they fighting against it (even if they want it) and finally giving in is just as sweet.
Add to the fact, like Snail said, it is a tactile exchange/experience. Touching is an exchange of emotion between people, whether hugging, kissing, patting a friend on the back, holding hands, etc. They all convey and emotion or feeling. I've somehow, somewhere picked up an appreciation for women's feet along the way. Not a fetish for sure, but an appreciation. The female form is fricking beautiful, period! This usually manifests itself in massages all the time which has NEVER been frowned upon by any female.
😉 So I can rub her feet through half a movie on the couch, enjoying the same tactile experience and her melting over it, in the same emotional/physical exchange as the tickling.
As for being just ler, I've spent my share of being an unwilling lee as a child and it was just fucked up. I don't in any way think its evil neither do I give anyone the thousand yard stare for it, and I actually enjoy it playfully. I'm just too much of a pussy to endure it hardcore like the ones of you that enjoy it.

I look at it like this, we're like a giant version of the kid's game with the wooden blocks that go through the similarly shaped holes. Some of us are a certain shape and only accept a certain shaped block, some a certain shape block that will only fit through the correct hole, and some shapes that can go through either. We're all part of the same game and love it, and love everyone else's love of it and the part they all play.