
There's the heart of it--risk. Mentors take risks on mentees and vice-versa. It's a symbiotic relationship entered into freely without any guarantee of success. Each person becomes invested in the other. Time, energy, effort. The mentor is rewarded with the satisfaction of watching a protege advance. The mentee advances--albeit, haltingly at times.
If you've ever been on the receiving end of the magnanimous investment of a mentor, I recommend you consider mentoring someone else. It's the golden rule in action.