If Rose is Mary Magdalene, what are you? A Goddess, written across the stars, in the household gods of Rome and the songs of the Ood. Your history is myth, and your kindness is legend. And you knew, once you'd taken Davros and his demons off the map, you knew what would happen next. The way it burns. But you're still human, and you were afraid. I'm so sorry for that. You didn't deserve the pain, or the fear, or the knowledge that you were dying. That any solution to the problem was a worse death still.
In the space of five minutes, the Doctor went from family, surrounded by family -- you, his closest, most faithful friend, and Martha, and Rose, and Jack, and all of them -- to completely alone. More alone than he'd been since the war. All of those whistling cracks and empty places were filled; he'd found something to live for. The TARDIS was singing her heart out, more powerful than she'd been in eons. And then it was gone, just like that. This man, who loves nothing more than to travel with the ones he loves, lost everything. He ends the season in darkness, staring out. Having killed the one he loved most.
In the moment he put his hands to your head, like a father to his daughter, like a husband to a wife, like annunciation, what were you thinking? That this is the last time you would feel this: this strength, this beauty and this power. The last time you would be anything, or mean anything. That he would take it all away, and you'd be left with nothing. Just some girl, getting on. Just a temp. Nothing special. Once you'd tasted it, Donna, how could you ever go back? That warmth around you, the power inside, the way even time and the TARDIS wrapped you in their embrace. You couldn't.
You'd go mad, like Caan, if they asked you to do what Rose did, and step across the Void. You'd touched too much of the world. You couldn't be allowed to remember it. We can never remember it. Every one of us has touched that, and every one of us has forgotten it. And the hole that it leaves is the song that we share.
The definition of madness is the inability to filter out the false from the true. But everything's true, given enough time and space: that's the difference between us and them. That filter is a biological necessity. If we knew everything there is to know, we'd die. The fact is, we do. Our bodies know better: they call it dreaming, and they force us awake. Stay there and become mad, become chimerical, abominable, something that cannot exist. The secret marriage cannot go on indefinitely: permanent direct contact with the divine is another definition of madness.