Physical attractiveness: while individuals do not rate this as important, research suggests it does play a major part. People are rated more favourably if with an attractive partner, and photos of attractive people are rated as being more popular.
Proximity: research suggests that one of the best predictors is how close people live to each other. Being in close proximity to someone is likely to promote friendship.
Familiarity: proximity also promotes familiarity and familiarity also increases liking for something or someone.
Similarity: couples tend to have similar interests or outlooks, similar psychological characteristics, e.g. intelligence and physical attributes – assortative mating.
Sense of humor- in another person increases the perceived benefits of a relationship (the pleasant feelings associated with laughter) and decreases the perceived costs (there is less likelihood that the person will become easily offended or burden us with negative emotional reactions).