The friendship, as well as all the feelings that we experience in our lives, is changeable.
In each period we are looking for, consciously or unconsciously,     what we pursue with the friendship.
In the adolescence we try to be together with other people to share experiencies in order to feel ourselves inside a group to feel warm and secure.
When we grow up the things change a bit and we try to find the confidence and the security in ourselves.
To sum up, I think that is not possible to define the friendship as a concept because it depends on what we are expecting of ourselves in each moment of our lives.
If you are ready to give love, confidence or support
 that is what you will find in your friends. The key is in you!