Harashima and Madara are children caught in the midst of their fathers' war. In Chapter 623, these two tighten their bond through mutual pain of the loss of their brothers. From that loss, they have foraged the desire to create a better place for their clan's children, a place which Harashima says would become the Leaf. Having each other gave them the opportunity to grow their desire from whim to full-blown hope.
At that time, the children had surpassed the hatred and customs of their opposing clans. The simple action of warning each other was treason to their clans. Even with the level of trust Harashima recalls, the reader knows it's not a question of whether they will remain friends. After all, this is the same Madara bent on destroying the village that shared the dream to create it. It's what the breaking point will be. Will Harashima's own father and brother be the harbingers of the friendship's destruction? With their family's pitted against one another, will the death of their loved bring about the rift between them? Will they betray each other, or use their ninjitsu against their clans?
The river of the ninja world is a churning and changing, and these boys are the skipping stones, tenuously crossing with little else but hope.