We can distinguish different kinds of friendships with different degrees of intensity. There are everyday social friendships, which are based on a clearly defined common goal. These can be divided into several categories. With one, you go to the movies; with another, you are on a mountain bike; you work with another person. You appreciate the humor of one person, the cheerfulness or the skill of the other. We could say that these friendships are more superficial although, of course, they can go hand in hand with a deeper friendship.
In The Little Prince, Antoine de Saint-Exupéry  writes about the little prince who goes in search of friends. When the fox asks to play together, he replies that he cannot because he is not tame. “Tame” is a forgotten word, he explains. It means “to establish ties”, only in this way can you get to know things. People buy things ready-made in stores, but friends are not for sale. If you want to tame me, he says, you must be patient.
But you will sit a little closer to me, every day (…)
We shall need each other. To me, you will be unique in all the world. To you, I shall be unique in all the world... (…)
It will be as if the sun came to shine on my life. I shall know the sound of a step that will be different from all the others. Other steps send me hurrying back underneath the ground. Yours will call me, like music.
The friendship that the fox represents here takes time to grow and gives a feeling of affection and connection. Here, it is about a friendship that is nourished by an attentive exchange of feelings. This friendship touches the inner world.
The Lebanese writer and poet Kahlil Gibran  calls such a friend
your needs answered.
And he is your board and your fireside. For you come to him with your hunger, and you seek him for peace.
He continues writing
When your friend speaks his mind you fear not the “nay” in your own mind, nor do you withhold the “ay.”
And when he is silent your heart ceases not to listen to his heart;
For without words, in friendship, all thoughts, all desires, all expectations are born and shared, with joy that is unacclaimed.
He calls friendship
The disclosure of its own mystery.
Respect and loyalty are the pillars on which friendship is built. Respect for who the other is and for what still can become, must become, what lies hidden in him waiting. Faithful in the realization that everyone is in development, where sometimes words can and must be spoken that bring a hard truth. Mistakes also come to light and are made again, but in the conversation, they can lead to new discoveries and insights for one and/or the other. Friendship gives freedom in connection. In a mutual exchange, one's own choices are made, even if opinions differ. The keynote is understanding and otherwise acceptance. Friendship can grow and deepen with age. Usually, things get weaker as they get older, but in friendship, it's the other way around. It gets stronger the longer it lasts.
But friendship can also sometimes be immediate, from the first moment. It may surprise you, but you are not surprised. Somehow you already knew it. And you recognize it right now, something in that other person. A gesture or a sound. Perhaps it is very old, from the past, from before your time. An inexplicable destiny or soul mate. Inescapable. It happened to the French philosopher Michel de Montaigne . He writes about this in his Essays. Even before the first meeting had taken place, he says, there was already talk of friendship.
A friendship beyond personal reasoning comes by some ordinance of heaven. (…)
It was not one particular motive, nor was it two or three or four or a thousand, but an elusive essence of all this at once, which completely seized my will, causing it to sink and lose itself in his. (…)
I can truly say 'lose', for we had nothing left for ourselves, nothing that belonged to him or only to me. (…)
In fact, I preferred to rely on him rather than on myself. (…)
If I should be urged to say why I loved him, I feel that it can only be expressed by answering: Because it was him; because it was me.
Ancient wisdom says that when a person is faithful and loving to one friend, he learns to open his heart. Then he can also be this towards all people he meets because he has developed that quality in himself. Similarly, friendship can come from one side; it can also be rejected. Friendships can be watered down or broken once life takes a new turn for either one. But friendship also extends across time and space. Two friends who have not seen each other for a long time continue the conversation where they left off. We do speak of 'strong friendships', but at the same time friendship is extremely vulnerable, subtle too. An invisible, elusive fluid, a sound or vibration that draws people together. A similarity. A complicated pattern of lines running from one to the other. A wondrous fabric of old connections in which new ones are woven, where nodes light up here and there. Friendship increases happiness and it decreases misery, by doubling joy and sharing sorrow. It is of great and deep value. It is one of the values of the soul. Friendship may be seen as a property of the divine part of the soul. That part is the connecting force between spirit and matter. It possesses the power of harmoniously connecting all things.
In the letters that Marsilio Ficino  writes to his friends, he bears witness to the Hermetic vision that 'man is a great miracle'. While translating the writings of Plato and Hermes Trismegistus, he discovered man as a microcosm and saw how it was possible to restore the connection between God, cosmos and man. His letters show that friends who strive with the same zeal for virtue and restoration, who help each other to develop themselves inwardly, are indispensable to him. His letters also show how the aligned soul's desire in friendship opens up a new dimension of intimacy and insights. He calls friendship
a supreme harmony of two people who worship the like of God.
Then there can no longer be just two friends.
It must always be three: two people and God; He unites us into one.
In this light, the word “lose” takes on added value and scope.
Whoever will want to lose his life for my sake, 
who will want to lose his self in the light of the soul, for the sake of union with the divine, will find the Life. Working together on the basis of the light, with the light as a guideline, gives friendship eternal value. Ficino:
A friendship is lasting which is forged by God. 
 Antoine de Saint-Exupéry, Le Petit Prince [The Little Prince], Gallimard 1943
 Kahlil Gibran, The Prophet, Alfred A. Knopf 1923
 Michel de Montaigne, Les Essais de Michel Seigneur de Montaigne. [The Essays of Montaigne], 1595
 The Letters of Marsilio Ficino, Shepheard-Walwyn 1975
 Matthew 16:25
 "It must always be three: two people and God; He unites us into one." Letter 51 of The Letters of Marsilio Ficino, Shepheard-Walwyn 1975