This document discusses three theories of relationship formation: the matching hypothesis, filter model, and reward/need satisfaction model. The matching hypothesis suggests that equally matched partners in terms of physical attraction are more likely to form relationships. The filter model identifies social/demographic factors, shared attitudes/values, and complementary emotional needs as important. The reward/need satisfaction model proposes that relationships form and are maintained because they satisfy partners' biological, affiliation, dependency, dominance, aggression, self-esteem, and sex needs through direct and indirect rewards like operant and classical conditioning.