A: Because your brain is a fragile, easily-scare-able vessel that's constantly trying to kill you while you sleep. Like, literally.
Q: Can I stop having nightmares?
A: Ha! No, you can't. But you can learn to appreciate the existential dread that comes with every sleep cycle. It's like a free bonus feature in life.
A: Deal with it? You can't. You can only try to outrun it. Or, like, build a moat around your subconscious. That's a thing people do.