People make friends with other people when they spend casual time together. This is how our ancestors formed their relationships, because humans lived in small, close knit communities in which options for making friends were limited, so we just formed good relationships with the people who were around us. This is why it is so easy to form new friendships in school and university. Society locks you and your peers in a building for several years. You share similar activities but more importantly: similar schedules. Overlapping and fluctuating social circles form, naturally giving you regular face time and shared experiences with many different people. Time to find others with similar world views or senses of humor. Proximity can be more important than similar interests. One study found that in student dormitories, the distance between rooms was the strongest friendship predictor — living closer together meant a higher chance of becoming friends. Another study showed that being physically present in a class a lot, without saying a word, makes others more sympathetic to you. So the most important principle of making friends is to regularly spend time with people in the real world. This alone can make it happen automatically and trumps all other advice.