Here's the short, simplistic version of what I think might be going on.
When we connect with people there are certain characteristics we really attach - find value in. It's kind of a cross between a fantasy image and reality. (humans have great imaginations sometimes).
It's VERY common to also have multiple interests. We're complex creatures. But that's part of what makes someone special/attractive. Straightforward and predictable soon become boring.
So it's very common for people to cast a lover in a certain role that seems to suit them well and we find it totally alluring. It's a big part of that connection. But if they were to step OUTSIDE that role then they would damage the current image we are so attracted to !
Does this make sense to you at all ?
My guess is that you are not by nature the 'dominant/sadistic' type. Maybe you are the fairy type. Light, happy, airy etc. He loves that about you and can't begin to envision you in an opposing role. And my belief is that you shouldn't try to be ! Unless it calls to you naturally - which is somewhat unlikely. You really can't be both and be the best at it. They are kind of opposing forces.
Be happy you have your role ! It's special. Perfect it !
Let him find his darker needs from someone who is more naturally constructed that way. The two are complimentary - not conflictive.