Why are we ticklish?

Good question: Some are & others are not. Some very, others only mildly ticklish. Thought to an exaggerated sensitivity of nerve fibers associated with pain and touch. Interestingly, most ticklish areas, like the armpits, are not areas of greatest skin sensitivity, e.g. Fingers. Tickling probably causes ultra-sensitive nerve endings to secrete a chemical which causes the "shivering" reaction from being tickled.