Because I’ve internalized the idea that I should be working all the time. Why have I internalized that idea? Because everything and everyone in my life has reinforced it — explicitly and implicitly — since I was young.
Like others have expressed, I wonder if this is specific to Millennials. While I´m not a Millennial, I have definitely subscribed to the idea that I should be working all the time. Ironically, I think it may lead to less productivity, which then makes me feel like I need to work more. It is a vicious cycle. I´d be interested in knowing if others have found a way to break it.