Part of the show Using Viruses To Treat Cancers - Moira Brown
Apparently it's all to do with a small area at the back of the brain called the cerebellum. Scientists at University College London found that the cerebellum lit up when volunteers tickled themselves, but not when a robotic tickling device was used. They think that this killjoy area of the brain sends out messages to cancel out sensations that it is expecting. In this way we can ignore routine sensations like pressure on our feet when walking, but react when something unexpected happens, like stubbing a toe.