The model of complementarity explains whether "birds of a feather flock together" or "opposites attract".

Studies show that complementary interaction between two partners increases their attractiveness to each other (Nowicki and Manheim, 1991). Complementary partners preferred closer interpersonal relationship than non-complementary ones (Nowicki & Manheim,1991). Couples who reported the highest level of loving and harmonious relationship were more dissimilar in dominance than couples who scored lower in relationship quality. (Markey & Markey (2007)). 

Mathes and Moore (1985) found that people were more attracted to peers approximating to their ideal self than to those who did not. Specifically, low self-esteem individuals appeared more likely to desire a complementary relationship than high self-esteem people. We are attracted to people who complement to us because this allows us to maintain our preferred style of behavior (Markey & Markey (2007), and through interaction with someone who complements our own behavior, we are likely to have a sense of self-validation and security (Carson, 1969). 

सामाजिक अभिसरण सिद्धांतSocial exchange theory
आकर्षण =मैत्री आणि स्नेह
समिपता/सानिध्य परिणाम Propinquity effect
पूरकता Complementarity