I liked this passage, from Michael Pollan’s easygoing expose
on the life of a modern beef cow:

It may be foolish to presume to know what a cow experiences, yet we can say that a cow grazing on grass is at least doing what he has been splendidly molded by evolution to do. Which isn’t a bad definition of animal happiness.

I think it’s a pretty good definition of human happiness, too – though we have a pretty hard time figuring out just what it is we’ve been molded to do. Maybe that’s what gets us into so much trouble – all the things we try that we’re not
molded to do.