Why shouldn't a person be friendly to everyone? Is "being friendly" and "being a friend" being differentiated here? Also, I consider a person as one's friend only if that person doesn't betray his/her other friends. So going by my rule of friendship, there won't be any problem if my friend is a friend to every person, since he/she is not going to give up on me and neither he/she will give up on his/her other friends when he/she us spending his time with me. What is the main context under which this proverb becomes true?