I've always been a fan of nightmares. I'm probably not alone in that feeling, but I'm certainly the only one that I've met in my life. I tell people about my fondness for crippling nightmares and the reactions generally span from "Uh, okay..." to "Dude, that's fucking weird".
I understand those reactions. On a cognitive level, I know why nightmares aren't a desirable event for people. Waking up in a start in a cold sweat, heart racing as the dim clock reads 4:38. Doesn't sound like a good time on paper.
To me, nightmares are the closest a normal person comes to death without actually dying. Or if not death, the closest you can get to your basest fears and still come out breathing. In my every day waking life, I am full of little fears and big ones, a lot of which feel intangible and un-targetable. A nightmare has distinct form and shape, a particular energy that grasps you.
At least mine do.
I've had one nightmare stick with me for my whole life. It's not the only one I have, and it's not that I have it frequently, but it does come in the circulation often enough. This is a nightmare that used to be the most terrifying to me as a kid, but with repeat occurrences it has become like an old companion, in a fucked up way.
I won't bother describing the nightmare here, because it really isn't the point. The point is that nightmares give you a shot of adrenaline and fear, challenging your psyche while your body is still safe.
I've talked with a bunch of people about dreams and dream significance. In my (very general) observation, people try to associate meaning to dreams on one level or another. When people associate meaning to nightmares, it's usually with regard to a struggle in their personal life- an impending interview, a falling out with a friend, yada yada yada.
I think we should just let nightmares be nightmares. In my estimation, there is more value in nightmares as shock encounters in life without meaning. It's mettle-testing, it's showing how, maybe, you'd handle the end of your life.
If you were to associate a meaning with nightmares, if you had to, I'd personally opt for one specific viewing. Think of nightmares (all dreams, even) as breaks in the walls that divide separate and parallel universes.
In the nightmares where you die, then that's a window into your own death in a parallel life. As absurd as the dream may be, it remains plausible to me that there is some universe where that absurdity is the norm.
When you wake up startled, feel the value in the life you have, in the safety of your bed. Know that for every light that goes out, the remaining light is just that much brighter. One day you'll be someone else's nightmare.