Do you underline passages in books when you are reading them?

As I read a book, I often underline or highlight passages that I find interesting, things that I can relate to, or words that are really insightful. When I re-read some books, I often reconnect with these passages instantly, but sometimes I find them odd, and wonder what state of mind or stage in life I was at when I first underlined them. Do you do this? Have you made any revelations about yourself when re-reading these underlined passages? Perhaps your tastes changed over time, or you have grown wiser? Does reading these passages make you want to refocus your efforts and get back into a frame of mind that you were once in?

Just for fun, I skimmed through my personal library and found:
The book with the most underlined passages: Henry David Thoreau’s Walden.
The last passage I underlined: “Thus we try to keep our heroes alive; hence we remember them.”