I started writing this as any time line, any of Doctor's and Master's lives you'd like, but as it worked out by the end, it fits some time during the Year That Never Was much better, or maybe sometime in the future, when the Master returns (as he undoubtedly will).
The Doctor never complained, he'd cover himself with a false enthusiasm, keep moving, always on a new adventure, always finding a new companion to drag along so he could show off the beauty of everything around them. The Master wasn't fooled, he new what went on in the other Time Lords mind, after all, nobody new the Doctor like he did.
He never complained because his wall would crack and it'd bring back all the hurt and pain. He wasn't so enthusiastic, he just needed a distraction from himself. He always kept moving because if he stopped his past would catch up to him, and never let him go. He always went on new adventures to find something which would show that the universe was beautiful and worth saving, rather than just the destruction and hate it shined with. He needed new people to show the good things to, because maybe if they saw the good, the bad would disappear. If someone could see what he could not, then he could delude himself into believing it true.
He was known across the universe, both loved and loathed. The Lonely God, the Dark Lord, the Lonely Angel, the Destroyer of Worlds. Funny how every time it was spoken with love, it was also filled with pity, lonely, even lesser beings could see the emptiness buried inside him. Over time, the Master had come to see all that was hidden behind that carefully constructed mask. Even the darkness. The deep blackness hidden in his soul; the darkness that had earned him the names of Dark Lord and Destroyer of Worlds. The Master whished to bring that out of him, he wanted to see the Doctor crash and burn, see the light leave his eyes and harden, cold and uncaring as slowly he broke apart, bit by bit. He wanted to destroy the Doctor utterly and completely, take all that he loved, leave only emptiness in its place. He wanted the Doctor to scream as he tried to hold up the walls, hold onto his sanity and shut the pain out. The Master wanted his pain on the surface. He wanted to Doctor to lose all control. He wanted the Doctor to hear the drums.
It would be so easy to break him, laughably easy, yet the Master never did. He knew the Doctor better than anyone else after all; he knew that if the Doctor ever lost control, nothing could stop him. The universe would burn.
So he never crossed the line, he pushed and tugged at it, stretched it as far as it would go, but never strained it so far it'd snap. Because if the darkness in the Doctor's soul came to the surface, the Master knew he'd burn right along with the rest of creation and time.
Instead, he'd watch the other Time Lord struggle, laugh at the pains he went through to not give in. All the while knowing the truth.
Deep down, the Doctor wanted to die. He doesn't travel the universe fighting for a reason to live; he travels the universe trying to find a reason to live.
It was pitiful in the Master's eyes, as he knew that the Doctor would never find what he was looking for, he'd carry on through his lives pretending to love and laugh and live, but then the time for death would come, and he'd finally breath a relaxed sigh, knowing he no longer had to run, that he longer had to fight or care.
And what a scary thought. The Master couldn't even begin to imagine a universe without the Doctor. How could he kill and rule over galaxies when the Doctor wasn't there to see it? What was the point of destroying if he didn't have his best enemy working against him, or trapped, having it all rubbed in his face by his Master. The dance couldn't continue with only one person. He always left space in his plans for the Doctor to break threw, just as the Doctor always left a crack in his plans the Master could enter, because if they weren't fighting, what was there left between them?
He knew that the Doctor cared for him, as he did him in his own way. Everybody presumed that he felt nothing towards the Doctor, how wrong they were. He hated him, but to hate you have to care, just as you have to care to love. If he didn't care for the Doctor, he'd have killed him centuries ago, or if not, then he'd ignore him, having nothing to do with him, the Doctor wouldn't even exist in his mind if he didn't care. But he did care, he cared so much, he hated so much, so much that the Doctor was always on his mind, that he had to track him down and hurt him, break him, watch him scream and bleed. You have to care to put that much hate into something.
The Doctor was everything to this universe, the love, the hate, the joy, the betrayal, the safety, the danger, the happiness, the anger, the spirit, the confusion, the God and the Devil. But the universe was nothing to him.
Something which the Master had spotted straight away with the Doctor is what he'd do to his companions. He'd mould them unknowingly into the things he hated. He'd turn them into his little toy soldiers, when he wanted them to find peace. He'd try to show them the beauty of the universe, but instead he'd show them the ugliness. He'd try to give them reason to live, and instead he'd give them a reason to die. Whenever the Doctor was in trouble, they were all tripping over each others feet trying to sacrifice themselves for him. They were all ready to die, and had no idea why it made the Doctor so angry when they tried to save him and the universe. The Master knew though, the Doctor didn't want anyone to die for him, he wanted them to live for him.
Often, on his trips to earth, the Master would here a saying humans seemed so fond of – "I'd die for you". If he was around the Doctor at the time, he'd see the other cringe. It was a saying both Time Lords hated, only for different reasons. The Master saw it as pathetic, giving your life to someone who'd happily let you die in their place. He also quickly saw the Doctor's reason for disliking such a 'noble' course. If you loved someone so truly and deeply then you could do so much more than die for them, you could live for them. After all, if they loved you so much in return, that too is what they'd want. If a person really did die to save another, then where does that leave their loved one? It leaves them living each day out in misery and depression, knowing that they're only alive because the person they love so dearly is dead.
Sometimes, the Master would wonder if he'd die for the Doctor, because he couldn't imagine never fighting their vicious dance threw the stars with him again.
He'd always imagined that the Doctor would out live him, because unlike the Master, the other Time Lord could go on without him. Yet now, he wasn't so sure that's how it would work out. After the Time War the Doctor had become so careless, so empty that he no longer cared what happened to himself personally. The Master knew he had to change that, get some reaction from him, if not fill him with warmth, then at least anger. So, he did all he could think off to rouse the other Time Lord. He seemed that much more alive when the Master was around, but the fire in his eyes was still out. Making sure to keep within The Line, the Master worked at getting his best enemy back.
He had to build him back up before he could knock him back down. He needed the Doctor to hate or love him, rather than not care about him.
He had to bring the Doctor back to life.