When we grow, we acquire new skills and broaden our knowledge. We gain new ways of seeing and thinking. And our experiences in life become richer. In short, we fill up. While filling up is a great reason to pursue growth, it's the act of pouring out that, I believe, is the best reason.
When you pour out (i.e. mentoring, serving, sharing, teaching, sacrificing, giving), you enrich the lives of others. That enriching, I've found, somehow, someway fills you back up. It's a sweet irony. Pouring out never leaves you empty. It only makes the world a more full place.