Perhaps it's just the unexpected generosity.

I remember a few years ago I was reading about positive psychology's various findings, such as that spending money on experiences with friends or family made one happier than spending money on an object for oneself; so I tried out buying donuts or bagels for a few university clubs. Everyone seemed much happier, for what was a relatively trivial amount of money invested (<$10 in every case).

(I don't remember noticing that the donut people were happier than the bagel people.)