purely by motivation, appreciation, and let’s be honest, the occasional kind word or warm smile

That’s the point why philosophers say there’s no such thing as “pure” altruism.
A warm smile is a small thing. A tiny little thing that costs no one anything. But we expect (excludes altruism) at least that tiny thing as a kind of “return” for our actions.

A brief thank you, a warm smile.
When at least that warm smile doesn’t come (anymore), even the most selfless person burns out.

Do we understand each other now?