In a rock-solid friendship, there is both give and take. There are mutual feelings between the parties and it's never one-sided. That doesn't mean that there won't be conflicts because we're human, after all. However, when there is a conflict between two very close friends, the struggles usually don't last long because there is forgiveness, understanding, and communication.
Forgiveness means that you let go of any malice or grudges and you move forward at peace with one another.
Understanding comes from knowing your friend inside and out - knowing where they're coming from and knowing what they're going through.
Communication means that you connect with your friend with an overall understanding of his or her relating style.