For some people, the physical reflex or reaction to being tickled may not be a pleasurable feeling, so they do not respond with laughter. According to MSN Healthy Living, studies also suggest that being ticklish may be a result of social conditioning.
Laughing as a response to being tickled by another person could be a result of sensitivity to touch. If a person's level of sensitivity is higher or lower than that of the average person, being tickled may be extremely uncomfortable. Therefore, some people may actually be ticklish, but it doesn't amuse them or make them laugh.
According to Dr. Robert Shmerling, senior editor of Harvard Health Publications, studies have suggested that people may respond to tickling by laughing because they've been socialized to believe that it is the correct response. It may also be a way of bonding between human beings, beginning with a parent tickling a baby.