Theories about

Theories-source: http://changingminds.org/explanations/theories/a_friendship.htm

 Factors that increase friendship

• Ben Franklin Effect: after helping another person we like them more.
• Contact Hypothesis: bringing enemies together leads towards friendship.
• Dilution Effect: information about others reduces stereotyping.
• Empathy-Altruism Hypothesis: if we feel empathy we are likely to help.
• Friendship: there are several factors that increase chances of friendship.
• Law of Attraction: similar attitudes leads to friendship. 
• Matching Hypothesis: romantic partners tend to be similarly attractive. 
• Mere Exposure Theory: exposure to people increases liking.
• Propinquity Effect: meeting and interacting increase chance of friendship.
• Reinforcement-Affect Theory: both forms of conditioning leads us to like others.
• Repulsion Hypothesis: we prefer those with similar attitudes (and vice versa).

Stages of friendship

• Social Penetration Theory: relationships go through stages of increasing familiarity.
• Stage Theory: relationships go through several stages.
• Stimulus-Value-Role Model: we select friends through a three stage model.
• Terminating relationships: relationships break down in stages.

Other friendship factors

• Attachment Style: we attach to others in secure or anxious ways.
• Attachment Theory: we attach to others in reciprocal and primitive ways.
• Buffer Effect of Social Support: supported people feel less stress.
• Communication Accommodation Theory: we talk/act like those we like.
• Love: there are several styles of love.