I've seen some fine points taken on this and as Orchid said
trust in anything is a key element and paramount to
making things work on both sides of the coin.

I've always believed that D/s has to be a melding of both
heart and mind. That willingness by both parties to take
pride in their given roles and do so because they believe
in each other and the bond they share between them.

Sometimes I think of it it terms of a submissive climbing
a mountain, they do so because they want to reach
its highest peak.

To overcome obstacles blocking their path on their journey
to reach that ultimate pinacle that with someone else
may have always been just beyond reach.

Most importantly knowing that should they slip along the way
they always have you there to catch them before they fall.