I can tell you 100% my toddler gets extremely frustrated when he can't do something by himself. And it wasn't a brusque development, babies get frustrated when they want to turn on the side and can't. Also, they're extremely joyful when they finally manage to do it on their own, and they'll do it constantly.

> doubt they even know what agency is

I doubt some people know what epicaricacy is, but they do it anyway.

Observed behavior. It's not hard to recognize feelings in one's child if one pays close enough attention.