Death is mocking him, ever so slightly, but he has something he wants. Isaac thinks for a moment of a snatch of poetry--because I could not stop for Death, he kindly stopped for me--recited in a child's sing-song in his mind, but he clears it and focuses. Too much detachment won't let him have a good death, and that's his focus now, it has to be.

"They stop you," he explains, "and you die."