For me, it's mostly a couple of things.
For one thing, I can get overwhelmed by the sheer number of stories available on Boolprop. I know I must be missing great stories, and I feel guilty about not reading more and commenting more, but unfortunately I don't have as much time as I'd like to. Also, there are a bunch of stories I've been reading for a long time, and some of them aren't really updating frequently anymore. Still it makes me wary of starting on new stories somehow: what if I take on new stories now, and in six months my old favorite starts updating again, and I won't have time to read/comment on it anymore? Will that person think I don't like their story anymore?
Another thing that likely contributes for me is that I've been around for so long. In the olden times, super detailed comments were in fashion. People would comment on different plot points and whatever crossed their minds. "So and so is being so mean, it's not fair to blame their dad for everything! Ooh, X is so cute after their teen makeover!" I kind of still tend to do this for the stories I read, out of habit and partially because that's what I've used to seeing as "polite". So even though a simple "I've been reading and I enjoy your story!" would make
me happy, it still feels like
I should be giving more detailed comments.
Oh and also, many stories have a bunch of chapters out already, so it can be a task in itself to get caught up. And then I feel uncertain about commenting while still catching up. I don't want to spam the thread after every chapter from three years ago, and would feel embarrassed if I then didn't actually fully catch up in the end and would never comment on the newest chapters.
So, silly things really. It's weird how easy it's to build these mental blocks on commenting. I quite like the idea of a heart emoji or something as a simple replacement of a "like" button.
It sounds like a nice way of showing support and encouragement for people's stories in a lighter way.