The Master had believed for centuries that he was over his idiot schoolboy crush. That it had died with his first body, or perhaps been crushed out of him by the force of the black hole.
He planned the Doctor's demise, but something went wrong. He told himself that he only fought to save the Doctor from the intelligence inhabiting the Keller machine because he needed him alive to carry out his revised plan.
But why, of everything he'd seen, was it the Doctor's mockery that the machine tormented him with?
And why did that image still break his hearts?