Research finds that the common factor in friendships is proximity. This means that people who are physically close to each other tend to become friends. Either because they do the same work, go to the same school, or have shared hobbies. This makes sense right? You can’t make new friends if you do not actually see them.
1. Commitment or loyalty
2. Mutual desire to understand each other’s behavior
- This means a tendency to accept aspects of the negative self-concept of the other.
- A willingness to be open and vulnerable (a willingness to risk self-disclosure).
- Attempt mutually to fulfill needs.
- Mutual modification of constructs, attitudes, and values.
3. Physical and social proximity
Physical and social proximity of available individuals with whom interactions can take place. This is obvious. But sometimes we forget that most friendships are formed in steady environments like schools, hobbies, or work.
“A general and consistent finding in social psychology and sociology is that of a high correlation between social interaction and similarity on such background variables as social class, ethnicity, occupation, sex, age, race, and education.”
4. Incremental or mere-exposure effect
Both the incremental model of friendship formation and the mere-exposure effect tell us that the more we see someone the more we like them. “Homans (1950) has similarly stated that “if the frequency of interaction between two or more persons increases, the degree of their liking for one another will increase.” Lott and Lott (1965) conclude after reviewing a large literature in the area that (numerous) “studies have supported the general hypothesis that interpersonal attraction is a positive function of interaction.”
However, the authors give a note of caution: “It is important to note, however, that while interaction over time provides opportunities for mutual reinforcement, it certainly does not guarantee they will be (either) positive or fully utilized.”
5. Shared stress
This is something we cannot really use to make friendships. But it has been found that intense, time-limited, and externally produced stress may be a powerful initiator of friendship. There are extreme situations such as confinement in prison, concentration camps, interactions during wartime, or times of disaster. Alternatively, (less extreme) shared-stress experiences such as those found in college, graduate school, and summer camp. It may be important that this stress is involuntary. This means you cannot kidnap a person to make them like you.
6. Perceived similarity
The perceived similarity is the most researched model of friendship formation. Research shows that people who find each other attractive, or are in friendships or marriage both perceive and are in fact more similar on certain dimensions than randomly matched people. Interestingly perceived similarity scores are generally higher than actual similarity scores. It has been found that more challenging relationships (e.g. long-distance) tend to also have more similarities. The similarity of status and equality seems to be important. Although, it is found that status is less important in friendships than in romantic relationships. In friendships, it’s more important to have similar values.
7. Perceived need complementarity
Friendships form because people fulfill each other’s unmet needs. Can you fulfill each other’s emotional voids?
Some research shows that the aforementioned might manifest in nurture and succoring or dominance and submission. Differences can be good if they complement each other.  If you like cooking and the other person dislikes cooking then that is a good match. In particular, if you like cleaning but they don’t.
8. Numbers Game
An Example: You want to become friends with a person named Sam. But Sam already has a full-time job, a small family, and a couple of friends, and regularly goes bowling with their bowling club. He simply does not have more room in his life for more people.
But you are not aware of that. The only information you are aware of is that he does not want to be friends with you. So you decide that either he’s stuck up, rude or that there’s something wrong with you. Forgive Sam, and yourself. He only has 24h in his day.
It doesn’t matter if you are the greatest thing since sliced bread, because Sam is simply unavailable. Relationships are complex and trying to make sense of it, although useful, can also be misleading and frustrating. Therefore, a good idea is to simply practice, and put yourself out there, a lot. Until you meet people, just like yourself, who in this moment of time are looking for someone to fill that friend-shaped void in their life.
9. Increasing Self-Disclosure
In different stages of a relationship, people share different types of information. In the beginning, when you’re just meeting people you might share superficial facts – my name is x, the weather is y, how are you? As you become more comfortable with the person you increasingly self-disclose intimate information about yourself. You might tell about future hopes and childhood dreams. If the person reciprocates then you are off to the races, because a closer relationship is forming.
10. Fighting Among Friends
Similarly, the reasons for friendship formation are the same for them to end. The reasons can be categorized: 
- (a) weakening of the shared stress situation
- (b) major time interruption
- (c) increasing perception of dissimilarity, and
- (d) increasing perception of non-fulfillment of needs
11. Physical attraction
Sprecher and Regan (2002) surveyed individuals’ romantic relationships, opposite sex-friendships, and same-sex friendships and find that physical attraction is most strongly associated with romantic partnerships, intermediately associated with opposite-sex friendships, and least associated with same-sex friendships.
- Fiebert, M. S., & Fiebert, P. B. (1969). A Conceptual Guide to Friendship Formation. Perceptual and Motor Skills, 28(2), 383–390. doi:10.2466/pms.1918.104.22.1683
- Campbell, K., Holderness, N., & Riggs, M. (2015). Friendship chemistry: An examination of underlying factors. The Social science journal, 52(2), 239-247.