Must the Dominant always be in control? Can’t they be ambivalence?

Yes, there is the desire for the Dom to always be in control because if he or she isn’t then the slave would be terrified.  (Like a child seeing their parent falter which then makes the world feel unsafe.)

I’ve seen and known many slaves who will leave a D/s relationship if they do not feel the Dominant is in control or feel the Dominant has lost control or doesn’t hold it tightly enough. Some submissives refuse to see their Dominants in a submissive role–for those Dominants that switch or have masochistic tendencies—they may be placing their D/s relationship in jeopardy. Most Dom with a masochistic need will feed that need privately (away from their sub) with another Dom.

It’s easy to forget Dominants are people too and they make mistakes—just as parents do—just as lovers do. Yet because the BDSM realm is one where mistakes can be physically as well as emotionally damaging and place life and limb at risk, the tolerance or acceptance isn’t always there for a Dom to screw up.  So Dominants tend to bottle things up and in essence keep themselves on a tight leash when necessary and when dealing with their own limits. It is not uncommon for a Dominant to stop a scene because the Dom’s been triggered; though they may not discuss why the scene was terminated.