I think that the best part of it is the satisfaction of teaching and training another, particularly where that person needs assistance. For example, A and B are D and S. -chuckles at the alphabet soup-

For A, if A were I, the satisfaction comes from helping B grow and develop as a person, and from the bond that develops between them. At it's best, D/s should be a positive and nurturing thing for both parties, helping them love and learn. And you know, get their rocks off...that's important too.