Because creating discomfort is part of the pleasure dom/mes get. Not merely phyiscal discomfort, but mental discomfort.
Putting you in a position you are uncomfortable about, but seeing you do it for me, as part of your submission, part of your devotion to me... that is part of the feedback I crave.
It's why we tend to push the envelope around the soft limits... and why we might talk or tease you about touching your hard limits... just to make you uncomfortable.
One woman who had a hard limit about public play got to spend a day sitting with me at a diner while I whispered in her ear "but what if I did put my hand between your legs right now? How quietly can you come if I insisted?"
A whole afternoon of public decorum, except in her head.