I realized after making my last post that it felt a bit incomplete. I have some more thoughts on the subject that could use some expansion.
I believe the most important factor in the growth of mental sadism is time. Trust comes in at a close second. The changes that occur in both Domme and sub must be nurtured. Standard interactions require time to become ordinary. The freedom for a Domme to experiment and process what is going on within her takes time, confidence, and communication. The sub must trust her enough to subject himself to her whims. She must trust that he can endure what she throws at him.
As things progress, symbolism often takes on a larger role and what an act represents often outweighs the act itself. e.g. to kneel and bow takes relatively little effort. The mental reinforcement of difference in status between Domme and sub ends up being the more important part.
Since well... nearly all of the responsibility for pushing the intensity forward falls upon the Domme, the desire to go farther is heavily dependent upon her growing appreciation for greater symbols of submission and devotion. This is how it touches her heart. I don't pretend to understand the sexual connection of sadism, but I believe things connect when she understands that emotional suffering is as powerful of a tool as physical suffering.
The end result here is that the growth of her sadism always leads, his emotional masochism follows.
Things progress, step by step. Make him feel more helpless, take away more choices, force him to do more things he doesn't like, limit his pleasure, restrain him tighter, and increase his suffering.
After say, 10 years of the lifestyle, it becomes a lot more clear how couples reach the point of prolonged chastity, cuckolding, feminization, TPE, branding, and the like. It doesn't always happen, but the elements seem to always continue down a path towards more extreme events... which keeps things fresh and exciting for both Domme and sub.
There is one important boundary that I think should be considered carefully when reaching the extreme end of the spectrum. I believe a sub can eventually endure just about anything... if he knows the Domme will never leave him as long as he remains loyal, loving, and obedient. This may just be my own block on things, but I tend to believe that if he feels he could be easily cast aside, the insecurity and fear of this act could bring about some severe emotional damage and not desirable in any way. I know in my own case, this is the line where my desire for emotional stress stops and it becomes more of a panic induced neurosis that isn't fun or beneficial for anyone.