Leon Festinger (1919-1989) was a psychologist who developed the theories of cognitive dissonance and social comparison theory.
Cognitive dissonance theory asserts that people can have two conflicting or inconsistent cognitions or beliefs which produce a state of tension or discomfort (also known as "dissonance"). People are then motivated to reduce the tension by reducing the dissonance, often in the easiest manner possible.
Social comparison theory proposes that individuals use evaluations of themselves against others to provide self-worth. Individuals compare themselves against others in various ways (like SES or physical features) and use these evaluations as a way to assess themselves.