This is second-hand, so bear with me. I don't have the author's name or story title, just a description: A person loses a loved one (not even sure about genders!) and an angel arrives to explain why that happened. He takes the guy to see what the future would be if that loved one had lived and it turns out to be horrible for the world. So it was better that the loved one died.
Second hand info says that the story was by Mark Twain, but I've been through his novels and short stories and didn't find it. Any leads or ideas?