I've never felt comfortable marking in books. I like to try to keep them in good condition and I wouldn't even know what to mark if I tried. I'd probably just end up underlining the whole thing which is as effective as not underlining anything. If I'm studying for school I'll write notes in a separate notebook and jot down the relevant page numbers if I feel I'll need to re-read the section.
Whenever I have a pen in my hand, I can never decide what is actually important enough to underline. Thus, I underline probably too much. This is especially apparent in my school books. I'm scared to try and reread my copy of "The Brothers Karamazov" because I think it's riddled with unnecessary underlines and jotted notes.
I've always been bad at distilling information down into concise, neat points. I'm bad at organization in general and even my own thoughts are hard to narrow down. A long time ago a close friend said I turn everything into a novel. I think that's part of why I don't know how to underline sparingly and am wary of marking in books. Too much of the context seems important to me and not just the big points that others might highlight. I often have trouble discerning where the context ends and the actual main point begins. It's something I've been working on for a long time but I think learning how to be concise will be a lifelong struggle, including in my note taking. For some it seems to come more naturally than for others but at least being conscious of it I can work around it better than when I was younger.