I can't even explain how sad this movie is (spoilers)
It may not even be the film but the sentiment that it produces. I saw it first as a mostly boring movie with very little dialogue and then realized upon watching it more closely that it communicated to my deeper consciousness and produced a level of grief in me I thought I couldn't understand. This is the loss of a persons personhood - you see a man relegated to a faceless ghost after a car accident (which we are all vulnerable too). All his physicality and identity stripped away. He then watches his life without him, travels through time forward and backwards and then to the time just before he was killed observing his living self. Then at the end after finding a note his wife left in the wall for him he disappears - free from the shackles as a tormented ghost walking the earth.
An incredibly sad film.