In social psychology and sociology, the idea that social change and stability represent a process of cost-benefit analyses between parties. Social exchange theory is kind of like the mathematical and logical side of a relationship. We add and subtract points for the following: how much effort are we putting into the relationship versus how much effort our companion is contributing, what we feel we deserve in a relationship and how likely is it that we could find a better relationship.
It should be noted that this balance-based theory is not only about romantic relationships. Someone might feel that their job takes advantage of them and therefore that person might go out and find another job if they think A: they deserve it or B: they could get a better one for the same amount of effort.
Example: According to social exchange theory, a person who feels that the cost of his or her romantic relationship outweighs its benefits will most likely leave the relationship.