What is the saddest sci-fi book you've ever read?

I'm looking for some recommendations for books that deal with sadness, existential dread or that generally veer more on the depressive side of things. I'm super interested in all nuances of human psychology in fiction but I've rearly come across books that have truly dealt gut-punches to me once I turned the final page.

Feel free to leave a little no-spoiler explanation next to your suggestions, I'm looking forward to them!