As aforementioned, nightmares are often very vivid because they're so emotionally charged. Though as Ellis notes, we can have very positive vivid dreams as well. Lucid dreamers, for example, can use their lucidity in dreams to master skills, flex their creativity, and explore their subconscious—and it will all be very vivid, because of that lucid state.
And of course, the scariest dream you've ever had was probably pretty vivid, and you'd likely think of it is as "bad" dream. Though according to Ellis, Laz, and Loewenberg, you're better off thinking about your dreams as messengers than inherently good or bad.
"If your dreams are intense and upsetting, I always suggest paying attention to them, as they are clearly trying to get your attention," Ellis says, with Laz adding, "Dreams help alert us to what is occurring beneath the surface of our conscious minds, and through these stories, we can later reflect and bring unconscious material and make it conscious—which can help us to feel better."
In fact, Loewenberg says, the more upsetting and intense the dream, the more important the message. "Dreams are a conversation with the self—a subconscious reaction to your current circumstances, behaviors, goals, struggles and relationships. So an upsetting, vivid dream is similar to you sending a strongly worded letter of concern to yourself," she explains.