The philosophical answer to why we can't stop daydreaming about adventures gone by. "Travel is a commodity, a privilege, and a state of mind; a comfort to some and a trial to others." It's the one thing most people around the world weren't able to do in 2020, and one that everyone's hoping to do in 2021. But, how much do we know about the psychological reasons we do it? Lorraine Boissoneault contemplates.