The manner in which gandhi reacted when he perceived that he had violated an oath suggests that may play a strong role in motivating some individuals to keep their verbal commitments.

I believe the answer is: avoidance of guilty feelings and negative reinforcement.
The avoidance of guilty feelings is a defense mechanism because guilt could often lead to personal stress. 
The negatve reinforcement under this situation refers to the effects that might cause a certain decrease in our standard of life (such as negative treatment from other people or some physical harm)