Answer: When we first met a person, variables like attaction, similarity, complementarity and proximity makes us want to have a relationship with him/her.
Psychological studies shows that people are attracted to similarity people: in the way we dress, talk, walk, in cultural and social aspects, when they have the same tastes for music, food, politics or even simple ideas.
Metaphorically speaking, birds with the same plumage fly together. We can assign the law of attraction for this meaning, developed by Byrne in the year of 1971, concluding that people's attraction is related with the proportion of similarity actions associated to another person.
When the relationship of two people are more established, the variables mentioned above still matters, but, time after time, the subjects will want to know, more deeply, the specifics of each possible branch related to the four variables: attraction, similarity, complementarity and proximity.
