As one of the leaders at MentorCloud, I get occasionally asked this question - what's in it for the Mentors? I think this question should never arise if a person has ever been a mentor. Just this week alone, two of my mentees made my day - one of them successfully completed college and received three fantastic job offers. Another is getting ready for round 2 of a business plan competition at MIT, to launch his 3rd social venture. Just watching them grow by being there has been an incredible satisfaction for me. I learn so much from the questions they ask, the situations they encounter, the fears they express and the aspirations they have. The ROTI (return on time investment) of being a mentor is priceless.
So, the best way to answer this question is to be one - a Mentor.