River knows that the Doctor loves her (well, ish, because neither of them can actually tell if what they feel is real, or if it's a result of what the other has been telling them since they each first met). Really, she knows it.
Which is why it hurt so much when she found a picture of a dye-blond human, hardly of age, standing next to her Doctor (okay, maybe not her Doctor, in bow-ties and tweed, but still, River thinks of all the Doctors as hers), the pare of them looking very much in love.
What hurts the most isn't even that he loved someone before her, that she's not the first (it doesn't make her any less special because she's Melody Pond, and even without the Doctor and the Silence, she would have been special). It's the fact that she's never even heard him speak about this girl. Not once. And it gets her angry because who's to say that the same thing won't happen to her too, when she's gone from his life, and he finds someone new?
So she calmly strides (but, mentally, she's running, in a panic, because, who on God's green Earth could possibly have meant so much that he won't even say her name?) to what is currently the second floor parlor, where he is going for a swim, and she asks him.
He looks up grinning, until he sees the picture that she's holding in her hand. For a single millisecond, something changes in his eyes, making it seem like he's reliving a memory, before he snaps back to the present, and gives her a the Oncoming Storm look. Even River has to shiver at that.
"Where did you find that?" He practically growls.
"I was reaching for a book, and it fell off of a shelf. Who is she?" And now River wants to know that answer even more, because, if this woman who is (or was) just a child to the Doctor, can make him act like this, then River is torn between wanting to rip her throat out, shake her hand, and curl up in a small ball to weep.
"Now, River," he begins to attempt to avoid the question, "I hardly think that–"
"Sweetie, tell me." And her voice is all threat and danger.
"Rose." He says after a long moment, "Rose Tyler." He stops there, like her name should be enough, like, even from that small, little glimpse, River should know who she is, and, maybe even love her. There's a wistful quality to his voice, too, which seems to say more about his feelings for this girl then words ever could.
"You loved her." River states the obvious.
He licks his lips before answering, "Well, she was human, so it wouldn't have worked out. I would have had to watch her die, and, so, obviously, nothing ever happened." Which, River knows, is 'Doctor-Speak' for yes.
"Do you still love her?" The question burns on her lips, and leaves her tongue feeling thick, like she's about to throw up.
Again, he pauses, like he's thinking, and then, finally, he offers her an answer, "She's in an alternate universe, now. With her family. And a human version of me, well, the old me, any way, that me," he points to the brown-suited, crazy-tied, modern-haired previous incarnation of himself. "It doesn't matter." He sounds all of his near one thousand years, as he says it.
By that answer, River knows that that was just another 'Doctor-Speak' yes.
She gets up and leaves him to his swim. She still loves him, and she still knows that he loves her too (because, he will lie about almost anything, but not that). But she also now knows that, if, by some miracle, this Rose Tyler came back to the Doctor, he would forget all about his River (the one that cleansed him of what he was making himself become) and run back to his Rose (the one that dried up and turned to dust, right before his eyes).
She almost cries at that.