I comment when:
I feel I can add value to the topic of the post
when something I have read stirs an emotion (positive or negative)
when a post or a picture makes me LOL for real
and when I can tell that someone worked like a f-ing dog on their post and deserves feedback INSTEAD OF HEARING nothing but CRICKETS in the distance.
I upvote comments and try to read through all the others' comments. It's how I learn on this platform.