Future. Time. Seconds. Double heartbeats. Infinite. Fight. Flight. Procrastination.
That's what was associated, in the Doctor's mind, with the Master. Somebody coming back often enough not to be utterly surprising when he finally appeared, certainly generally distressing, but not totally unwelcome. The Master was stolen hope and stubborn denial repeated so many times it had become a fixture into the Doctor's long life, a basso continuo thrumming seductively at the back of his mind.
There was the matter of the Time Lock, of course. Not that he couldn't trust the Master to find a solution even better than Rassilon's (though, perhaps, not less disruptive or grand) to evade physics and come back to him. (How silly it was the thought, that the Master could come back into the larger Universe just for the Doctor, and not for power or other inane quests.) Of course he'll come back, his lucky bad penny, he couldn't leave him on his own- and then-
The one person that the Doctor can't avoid, or smother in half truths and patent lies is himself. Then (if, then) nothing new would happen, surely, nothing would change, and no words would express anything new, or old as their lifes. He holds that for certain, but nothing can make his hearts beat a jollier tattoo, each bar closer to the Master's return, four at a time.