After doing stupid amounts of research for my last story, here are some facts:
The areas of the cerebral cortex associated with the sensation of touch (which is the somatonsensory cortex), and the pleasure center of the brain (the anterior cingulate cortex), react much more suddenly to a tickling stimulus delivered by an external agent.
There are some spelling errors in there, I'm sure, but I can't be arsed right now to spellcheck.
In short, depending on how the touch is delivered (nice, quiet, dark room with some lit candles - your sig other gently strokes the back of your neck or whatever), makes you feel good. That same person in the same setting starts to tickle you - it also makes you feel good but a whole lot faster and all at once. And, since pleasure is along the same lines of pain, it makes sense that a lot of people react too fast depending on the setting and mood, to be able to tell the two apart - hence, they dislike it automatically.