I laugh because I am greatly amused. During sessions as a 'ler or as a Domina with my submissives, I am constantly giggling because I am amused and delighted. Why do what I do if I don't find delightment and amusement in it?
With the tickling, I know a good portion comes down to that I totally understand how the 'lee feels, bound and helpless and tickled senseless. Its thrilling. Then much of my amusement as a 'ler comes from the sounds they make, they way the jump and squirm when I hit the right spots and the faces they make.
I also giggle because I am getting turned on. When I tickle someone, I prefer them bound. I delight in the power exchange that bondage brings. I get drunk on control. And I get wicked. I tease and taunt and I laugh at them in their state. Begging and squirming like they do only fuels my evil giggles...
In summary: Why do I laugh when I tickle another? Well, simply, like Slavetickler pointed out, I'm an evil girl form another planet and I get off on tormenting others!
*insert evil anime girl laugh here*