While listening to the Doctor Who season six soundtrack, I was struck by the title "Loving isn't Knowing (The Almost People Suite)." At first, the title seems not only odd, but even paradoxical. Most of the other titles are derived from dialogue ("Run, Sexy," "Always with the Rory") or a situation ("Chemical Castle," "The Impossible Astronaut"), but the significance of the title requires more thought.
In context, it's a reference to Amy's ganger, but the wider context applies to both of the Pond couples. Most people associate love with knowing everything about a person--even the Biblical "to know" implies intimate, physical knowledge of another person.
But what is River's catchphrase? "Spoilers?" And in Angels take Manhattan, she says "Never let him see the damage." The Doctor and River keep secrets from each other all the time, secrets of identity and love and death. Amy is much the same; in both Pond Life and Asylum of the Daleks, she refused to tell Rory why she was pushing him away. Perhaps that's part of the reason this song is so sad. If we can never know anyone else, what does it mean to love?
"Trust me," River told Ten. "One day I'm going to someone that you trust completely." She then proves it by whispering his name in his ear. Given the Doctor's timey-whimey lifestyle, perhaps that is the only he can interpret love--as something he can trust, not something to know.