What's the PURPOSE of friendship?
To the world you may be one person, but to one person you may be the world.- Dr. Seuss
The core PURPOSE of friendship is to help us change and grow. I can't emphasize enough how important good friends are to your happiness and success.
The problem is we're uncomfortable with friendship having a PURPOSE. The more we define friendship, the more we can focus on what people should be in our life—and, more importantly, the ones we shouldn't.