We go through life forming all kinds of relationships; with our parents, siblings, friends, acquaintances, and finally we may find a lover or our future spouse, through interaction with these people in our daily lives. However, we often find that relationships become disagreeable after a certain period. What makes a relationship last? There is extensive research being carried out on this subject by social psychologists, who have concluded that there are a number of factors that influence us in choosing friends, falling in love and in establishing and trying to maintain close relationships.
In order to understand why relationships last, we need to first understand why we like the people we do and why we dislike other people, and why we feel indifferent towards many others that come in our life.
Essentially, three factors govern the beginning of any kind of relationship - Physical proximity, Emotions and a need for affiliation.

Physical Proximity
People, in their lifetime, meet many strangers. When these perfect strangers are forced to meet each other on a regular basis, they become acquainted. Strangers may sit next to each other in a classroom or examination hall, may rent an apartment across the hall from one another, wait at the same bus stop at the same time, or work in the same office.
These circumstances lead to a degree of closeness, which gradually increases to familiarity, and will result in two strangers exchanging greetings after only a few meetings. Therefore, any aspect of the environment that increases the physical proximity of two individuals on a regular basis increases the probability that they will gradually get to know and to like one another.
However, for these repeated exposures to develop into friendship, it requires a positive attraction. When a stranger acts unpleasantly each time you are together, it leads to intense feelings of dislike, which may cause you to avoid the person altogether.
Emotions
All positive and negative events in our daily lives can have immediate and intense effect on our general mood. Mood, in turn, affects many aspects of behavior, including interpersonal attraction. The positive emotions will increase interpersonal attraction, while negative emotions decrease attraction. In simple words, we tend to like people who make us feel good, and dislike those who make us feel bad.
Need for affiliation or Friendship motivation
Every person has different levels of motivation for developing friendships. Some people prefer to be alone much of the time while others behave in a very sociable manner. Such personality differences help to determine whether physical contact and positive emotions actually lead two strangers to interact.
After a relationship is formed, those individuals with a high ‘friendship motivation’ level are increasingly satisfied, as the relationship progresses, in the first few months. Those with low motivation levels, tend to feel less satisfied over time, especially if they have a high need for independence.