The most interesting theory I've heard about why people are ticklish is an evolutionary argument. The argument was that it was something people developed to train ourselves to fight. The theory is based on the fact that many ticklish areas are concentrated around areas that would be very bad if we were injured there. For example there are major arteries in the armpits, thighs and hips. Your ribs protect many organs, your neck is important, stomach/sides are a soft spot, and without our feet we'd be in a very bad way. Now how this developed is it teaches you to protect these areas, and since it makes you laugh people will do it. For example I have one friend who immediately starts crying if she's tickled, so needless to say people don't find tickling her much fun. So the fundamental idea behind evolution is high survival value of a trait like being ticklish means more people will be ticklish, and natural selection will weed out those who aren't. It's believable to me that being ticklish had a high survival value ages ago because when from a young age you're trained to protect the areas that every other person wouldn't necessarily protect (head, centerline etc.) you have an edge, and you also know where to hit to do damage. Now the holes in this theory could be that the head and chest aren't the most ticklish parts and things of that sort. We also don't know if ticklishness is at all genetic. Either way, this to me offers a pretty feasible explanation of why most people are at least a little ticklish. Now for you and your ticklishness (or lack thereof), the exception, it just seems that according to this theory you came from a few rare genes that slipped through the cracks of natural selection. For this theory to be true we have to assume that ticklishness is genetic and that people do learn to protect these spots from being tickled. I don't think this is something we'll ever truly know in our lifetimes as it's not exactly on the cutting edge of science, and I doubt anybody's discovered a "ticklishness gene". Sorry if this reply is bad news to you, just something I read when I wikipedia'd tickling, and it makes enough sense. On the other hand, my girlfriend has a pretty big mental block she developed to deal with pain (long story) and when she's tense or in a defensive mode there's only a couple of spots I can get her in, otherwise the more she lets down her guard, the more ticklish she is.
