How can you call something that's done 'not at your expense' and done 'willingly' a SACRIFICE?

Sacrifice, by definition, means giving something up, typically for the greater good.

To feel you have sacrificed means you feel you have given something up.

To GIVE for the greater good is not a sacrifice. It's missing the operative word, UP.

Marriage is the triumph of imagination over intelligence. Second marriage is the triumph of hope over experience.
(Oscar Wilde)