In its rightful form, friendship is a relationship which is relatively non-hierarchical and less susceptible to possessiveness. Many other relationships, which may not have started as friendship, but which grow deeper and stronger with time, start displaying the qualities of friendship. Healthy husband-wife or parent-child relationships for example can mature into beautiful friendships. This is only natural as the etymology of the word "friend" in various languages can be traced down to indicate "to love" and "to join". To form and sustain true "essence" friendships, one necessarily has to sacrifice a portion of one's egoism. More someone can give up egoism, more space is created for friendships.
Most of the above pertain to friendship in its ideal form. Such friendships are more likely to be formed in cases where stable shared values form the connecting factor for the relationship. However, stable shared values are not common in today's world. What brings people together more often are shared interests. Interests are often transient; and as they change, so do friends. Sustained interests can lead to sustained friendships - but if such interests are not anchored in some deeper values, the friendship remains at a shallow, superficial level.
All the mature ways of self development I have encountered are unanimous in their verdict : real self development is accompanied by increased connection with others. And from practical descriptions of such connectedness it becomes clear that the relationship involved is predominantly one of friendship.