Gain-loss theory of interpersonal attraction means an attempt to formulate a theory that takes account of changes in people's liking for each other. It suggests that increases or decreases in the rewarding behavior we receive from another person have more effect on us than a constant level of liking or disliking. Thus we like best someone who starts out negatively in our estimation and becomes more positive, and we like least a person who starts out positive and becomes negative.
Definition