Greetings, I recommend you the excellent book by A. C. Grayling "Friendship (Vices and Virtues)", from which I extracted this quote:
“The idea of seeing friendship as a relationship in important part predicated on wishing the good, and promoting the good, for one’s friend, and of this itself contributing to the good of one’s own life, is surely part of what we have to mean by friendship, and although it seems, once it is explicitly stated, an obvious enough insight, it is too central to be treated as merely implicit.”
A.C. Grayling