They do not have hearts.
They have no loyalties. No obligations. No attachments. Their oaths are ruthless, bloody, their promises like a shark's smile. Sharks, yes—they are like sharks. They do not close their eyes, not even when they sleep. They are drawn to blood, but are not bound by it, and trying to bind them with something even more ethereal is like trying to catch smoke in a butterfly net. They may be cold and hard, fiery and burning, crackling and snapping, howling and snarling, silent and watchful, but what they are not is compassionate. They are not caring. They are not humane.
They do not have hearts.
So why follow him? He created them, but that has as much worth as a leaf floating in the ocean. Why follow this man who would be god, this shining leader? He plays with peoples' hearts, toys with them, but they do not have hearts, just holes, and he is not god yet. He can not manipulate empty space.
Aizen does not have fear.
There is something missing in him, but that isn't a bad thing. He has cut away all his fear, and his vision is clear, unobstructed; his gaze is sharp enough, focused enough, to cut. And without his fear, they can see him, too; they can see the white-hot core of him. They can see the part of him that i will /i become god. So they step forward, and they kneel, and they bow their heads and they promise to serve him forever, because that light is drawing them onward with its promise, its potential.
Why? Why bear the brunt of his world, his dreams, his visions on their shoulders?
Because they know that he will find them a place in his new world. They know. They just know.
For they do not have hearts.
But they have instinct.