I have a recurring dream about an attic that my grandmother doesn't have. I usually go up there as my family and relatives have dinner and I don't want to come down because it is filled with memories of my childhood and past....the items are usually different, and this dream almost always end in a nightmare. Tonight's nightmare was an apocalyptic world where big asteroids fall from the sky and kill people. I also dreamt there was a mad hunt for rare items worth a lot of money but no one knows what they look like.

Why do I dream about a fictional "Grandma's attic" and why is it usually followed by a nightmare?