Adlerian individual psychology
To understand more about the book I am reading – 嫌われる勇気, searched a bit about Adler and his theory. Found this link is quite helpful Adler and Adlerian individual psychology
Anyway, human is a social being. Human’s exist is inseparably linked to the existence of others. Thus, all of problems / unhappiness are rooted from his / her social problems.
Now, going back to the last part of the book.