The author identifies 7 pillars of friendship. This is super obvious looking at many of my core friendships, but had never thought of it in a scientific way. 1. Growing up in the same place, especially during the core teenage years. 2. Speaking the same language. 3. Having the same education. 4. Enjoying the same hobbies. 5. Sharing the same moral or political point of view. 6. Sharing the same sense of humor. 7. Sharing the same musical taste.