That's a great question. If we laugh when someone else tickles us, it seems to make sense that we could tickle ourselves, but I've never known anyone who could. Basically, there are three important things going on:
1. Our bodies filter out unimportant information. Your body constantly gets messages about things touching you. Your clothes touch you, your feet may touch the floor or your body touches a chair. Your brain ignores this information most of the time so that you can concentrate on important things.
Bugs crawling on you have a very light
touch, so normally they might not get much
reaction from our brains. It might be very
important to know that there's a bug on you, so
maybe that's why we evolved a tickling sensation
that we really pay attention to. If someone
tickles you, you try to escape or get rid of it. Maybe the tickling response makes us try to get rid of spiders, ticks, or insects that might be dangerous.
2. Our brains pay more attention to surprising things. If you are reading or watching TV and scratch your nose, you probably won't even remember doing it. But if your friend scratched your nose, you'd be surprised and remember that.
When you tickle yourself, there's no surprise. Your brain knows you're going to try tickle yourself because it gives the orders to your hand. So it doesn't pay much attention. If you touch the soles of your feet, You feel yourself touching your feet, but there's no tickling sensation.
There's an interesting experiment that
showed how important surprise is. If people use a
robot to tickle themselves, they do laugh. The
important difference is that there's a very small
pause between the person starting the robot and
the robot starting the tickling, so even though
they are expecting to be tickled, their brain is
fooled because the touch doesn't happen when the
signal to move the hand is sent out.
3. We sometimes laugh when we are scared or
uncomfortable. According to the source I looked at, laughing is a response to a person being scared or uncomfortable about being tickled. If your brain knows that you're the one doing the tickling, there's no fear involved, so you don't "feel tickled." I don't know why some kinds of fear make us laugh and some don't, or why some spots are more ticklish than others.
If tickling is really part of a bug-avoidance system, we would expect other animals to be ticklish. Do you think they are? Thanks for asking,