Why some people are not ticklish?

Tickling produces a pleasant response in ticklish people, and as a, result they laugh. Some people have nerves that don't produce that pleasant response, and as a result, they are not ticklish. In such a case, those people might feel uncomfortable when tickled. 

A person's response to tickling will depend on his sensitivity to touching. A lower sensitivity to touching might prevent a person from being ticklish.

Some scientists believe that being ticklish is a conditioned response. According to this theory, a person must first learn to respond to tickling with laughter in order to respond later on in the same way. Some people could simply have not been conditioned to tickling.

Some people might be ticklish, but for the right response to happen, the proper area must be reached. In such a case, touching the wrong area might not lead to a laughter response.

The laughter response is believed to be a reflex made by the nervous system. If a person's nervous system is less sensitive, then this person might not be that ticklish. (See 20 Signs That You Are Emotionally Sensitive)

As people age, their touch receptors get reduced. Older people are thus less likely to be ticklish than younger people.

According to some experts, the ticklish response might be associated with the bonding instincts the child develops at an early age. This bonding style might determine how a person responds to touching later on.

Some people suggest that tickling results in a fear response and that the purpose of that response is to help a person defend himself from attackers. According to this theory, the less the anxiety response of a person, the less likely he is to react to tickling. (See The Body Language of Anxiety)