The final scene with Mal and River flying, when River asks him to say the words she knows anyway:
Mal: Love. You can learn all the math in the 'verse... but you take a boat in the air that you don't love... she'll shake you off just as sure as the turn of the worlds. Love keeps her in the air when she ought to fall down... tells you she's hurting before she keels. Makes her a home.
- Mal calling River a "little albatross".