At first it was, well, fun. A kind of art, the Master's overcomplicated schemes decorating the Doctor's blank and endless exile. A kind of game to fill the days.
But the Master's games are all one game, repeated. The Doctor's getting bored, tired of anagrams and disguises, taunts and offers. Tired of banter where there once was conversation. Games are poor things to make a life of: rulebound, empty. Games are ceremonies that mean nothing.
Everything they hated about Gallifrey, they've become.
And every time the Master smiles at him with fond bitterness, his hearts leap, and they begin again.