Someone else's fingers on your ribs make you squirm. Your own tickle far less.
When you move, your brain predicts when, where and how hard the touch will land, and dampens its response. It can't predict someone else's touch, so that feels much stronger.
In one experiment, people moved a robot that made another robot touch them. Delaying that touch, so it came later than the brain predicted, made it feel more intense.