Or I don't find unfortunate genetic arrangements (or accidents, or whatever else may render someone handicapped) funny. It's really no better than being a racist in my mind, since this is far more often than not something over which the person has no control. The exception to this would be something like a guy being rendered an invalid due to a drunk driving accident or some shit, and even then it's not something to laugh about.
"I have a fully functioning brain and body whereas you do not! Haha!"
Everyone has seen a goofy one. The kind that are at the grocery store and wig out about mellons or decide to talk to everyone in line or run through the streets going "YAY~!" I've seen those types and I'll laugh, I don't wish that upon someone nor to I laugh at those who are stuck in a wheel chair* and don't think you should walk up to a retard, point, laugh and give him a wedgie.
* Though anyone in a wheel chair going down an incline out of control is funny.