Here's a definition of spiritual friendship from Tim Keller that puts it so beautifully, and gives us something to aim for. (He's writing about marriage, but I think it applies to any friendship.)
Friendship is a deep oneness that develops as two people, speaking the truth in love to each other, journey together to the same horizon. Spiritual friendship is the greatest journey of all, because the horizon is so high and far, yet sure – it is nothing less than “the day of Jesus Christ” and what we will be like when we finally see him face to face…What, then, is [friendship] for? It is for helping each other to become our future glory-selves, the new creations that God will eventually make us.
From Tim Keller The Meaning of Marriage 116-117, 120.
image is by Zaro from flickr