people always talk about leaving comments on ao3 like it's a nice thing to do, or the best way to encourage writers to keep writing, or overall like it's how you Do Your Part in fandom
and yeah, all those things are true, but having spent the past few months leaving enthusiastic comments on as many things as i can, i have a different perspective
you should leave comments on fics because it's fun
taking the time to stop and focus on what i like about a story has made me way more aware of what's going on in stories and what i like about them. there's bit more actual comprehension and appreciation and not just beaming content into my eyes to fill time
i like noticing cool little things in fics, or riffing on funny events. i've never been very good at speculating or picking apart characters, but sometimes something clicks and it rocks.
and of course it's pretty nice when you get a response and it's clear you've made another person happy
so yeah, you should leave comments for your own sake, too. it makes reading better!