Those two words could rattle his mind, his entire existence.
"The Doctor does not fall in love. No siree." He repeated it to himself like a mantra whenever he heard those two words.
Those two words, only when spoken by the great River Song, could bring nations to their knees. For they knew what was to come, the great Doctor (of course accompanied by the wrath of River Song, they often couldn't figure out who they were more scared of). He just so happened to follow River around like a lost puppy dog whenever he heard those two words.
But he wasn't in love, of course not.