1 They don't get a pleasant feeling
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.
2 Lower sensitivity to touching
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.
3 They were not conditioned to tickling
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.
4 You are not touching the right areas
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.
5 Less sensitive nervous system
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)
6 They aged fast
As people age, their touch receptors get reduced. Older people are thus less likely to be ticklish than younger people.
7 They formed a different bonding instinct
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.
8 Lower anxiety response
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)