Question: Why do we react to tickling?

Hi Trololo, great question.
We can sense if something has touched us because of receptors that are found in our skin. Receptors are different kind of proteins that react to different things, and are found in all kinds of tissues that make up our bodies. Different types of receptors react to different things. Some of them react to certain chemicals (like the ones in our brain and nerve cells), some of them react to being touched, pressure, vibrations, or to the hot or the cold, or to injury (like being/having a cut). When receptors are activated, a message is sent to our brain through nerve cells, and then we know that we have been touched and where and how hard (like whether someone tapped you or punched you).

We have many receptors in our skin, all over our bodies, but we also have different amounts in different areas. The skin in more sensitive areas of your body will have more receptors than the less sensitive parts. So for example, the skin on your lipsor your palms have more receptors than you would have in the same sized area of skin on your arm or your legs.

Ok, back to tickling. I think the areas of our bodies where we are ticklish do have a lot of receptors in them, but just having a lot of receptors doesn’t make an area ticklish (for example, most people are not ticklish on their palms). Apparently the kinds of receptors involved in the tickle response are ones that respond to touch and to injury. These are activated when someone else tries to tickle you, and you get different receptors sending messages to your brain telling you that you are being tickled. When your brain gets the message, you react by laughing and I think this is a reflex. If you try to tickle yourself it doesn’t usually work, so it looks like some amount of surprise is needed for the tickle reflex to occur. Apparently, laughing associated with tickling is a nervous reaction that can actually be triggered and that some very ticklish people even start laughing before actually being tickled!