because my heart still sings the song of a thousand tongues
The first time the Doctor sees River Song, really sees her, is also the very last time he sees her.
He'd felt his chest twist a little inside when she'd told him she was a Professor now, but then there had been the Emperor who wasn't the Emperor but really a scary lizard-looking telepathic monster out to eat the thoughts of any living creature that came its way and a whole planet that somehow got itself kidnapped by another lizard-looking monster who was incidentally not telepathic—
—and somehow he'd just (conveniently) forgotten that she was now Professor River Song, Archaeologist until she'd demanded Durillium.
He couldn't say no again (rarely could say no to River in any case) especially when she told him her next job was coming up and she wanted an evening away before she went off to commune with old books and dust, so now he's sitting across the table from her in a very expensive restaurant and realization slams into him like a wall of bricks.
She's laughing, a real laugh, not one of her I-know-something-you-don't laughs, at the story he's just told, and for right now they're not the Doctor and River Song, Heroes of the Universe, they're just the Doctor and River Song, Old Married Couple.
It should be so ordinary, so domestic, the thought should bother him, but it doesn't. For once her realizes that River Song loves him not because he's a Time Lord, not because he's some rarity, not because he can get her out of prison anytime she wants, not because he's just as odd as she is so they understand—she just loves him because he's the Doctor, full stop.
And he realizes that he feels the same about her and has, perhaps, for a very long time.
He's suddenly struck with the urge to grab River's hand and drag his wife—his glorious, beautiful, brilliant wife—and lock her in the TARDIS and never let her out. But he knows he won't, because he's always known this was coming. It was always this way. They all left, one way or another, and it was just the madman with his box again. And he thought of red hair and blonds and of a half-Doctor and his wife living happily-ever-after and Rory and Amy both perfect halves of the other and he knows he can't—wouldn't—change his past, not ever, not one little bit.
So River must go to The Library, and he must carry on alone.
He should be used to it by now. It shouldn't hurt this much, shouldn't squeeze both his hearts like a giant fist—
But it does.