People who journal about books you read, how do you know what’s important enough to write down? What are your journaling/note taking/annotating habits?

For those who have a book journal to keep notes/thoughts on what you read in order to remember better, how do you know what to write? I write my thoughts and questions but I don't really have that much of them. When summarizing chapters I find myself bulleting nearly everything that happens. I feel like [...]