Do NOT tickle me if you value your life! I expect there can be several reasons. Some people are especially sensitive to touch in general. Being touched lightly in a tickle can make some people think of being attacked by bugs --like the sensation of a fly darting about their skin, or a more in depth tickle might seem invasive, especially to one's space. Some people might just plain feel undignified being forced to squirm about and giggle and laugh with no control over the situation.
