Mmmm well, for me, at least, I'll comment if I like the author or feel comfortable with them. I tend to lurk a little and see how they're interacting with others, especially those of my ilk.
The person the author is is important to me. If they're personable, kind and genuine, I'm more likely to comment - there's something enticing about having a rapport with folk like that. It's hard to describe, but it's there. It feels like you're rewarding them for their warmth and kindness as well as their work, which you'd feel is written with you and your fellow readers' enjoyment in mind. The better the author treats myself and others, the higher the quality of my comments. The longest single comment I've given this week on another site is 313 words - I've given that author a approximately 840 words worth of comments in all their four chapters, not including replies to their replies.
The breezy and fun types of authors who like short comments will get fun comments, puns and jokes.
As far as I know, I'm not alone either. I've spoken to other readers in other forums and they too have mentioned that they'll comment if the author is nice. Perhaps it's because writing on platforms have become a bit like social media, and there's a lot of competition to get people to engage with creations. You don't just sell your art - you sell yourself too. It's a bit like a package deal; an extra to stand out amongst all the other writers.
Having inserted images help too, on my end at least. I'm more visually inclined, so you'd expect me to at least say, "that's a very pretty picture!" or "I like what you did with the lines", especially if it's drawn by the author themselves. I'll notice a change in covers too, sometimes, even when unprompted. But it really depends on the reader. I've heard of those who don't like images with the text and prefer them placed elsewhere, in the notes perhaps, etc. So I would not readily recommend an author do it unless they're very sure they want to or it suits their style and format.
If it's critiques that the writer wants....... I don't really give critiques in general. I understand they'd like those to help them improve, but that sort of thing is not really my domain.
As far as stopping goes... I don't really comment further if I don't receive a reply. I've also a history of deleting comments I've made if I don't receive replies to them, especially if the author is replying to other comments but not to mine. It's mostly to save face. The latter doesn't really happen anymore, because nowadays I tend to lurk and pore through other comments and the author's socials or activity before commenting at all.
That's mostly it, I think. It's not hard for me to read at least one chapter and make comments on it. It's just that there are certain things that compel me to do it more or less. But the one thing I'd agree with is that the summary and premise of the story does have to catch my interest as well. It's a combination of a lot of things, really.