I've helped several people with nightmares, this is often a problem that develops over time due to the way our mind embeds negative patterns. A nightmare is usually linked to childhood or trauma and can also come from influences in our waking life that cause fear.

The reason why you are having nightmares is that certain fears have embedded and your subconscious mind is actually trying to get you to resolve and release the negative-pattern so they don't embed further.

Once you start to understand how thoughts replay in dreams from waking life influences you can start to chip away at those past-patterns replacing them with more positive dream experiences.

This is the part of dreaming I call house-cleaning or the wildness of the subconscious mind. If I can help a person with chronic night-terrors at the age of 30 no longer have them (the worse kind of nightmare as it activates the primus the nerve cluster in the cerebellum that causes fear paralysis).

The other reason to treat nightmares is often not understood by most and that is the nature of how dreams connect to the limbic system causing a hormone response ie the fight-or-flight mechanism is tripped so the mind releases cortisol and adrenaline during a nightmare causing the body to start to have elevated heart-rate, constricted blood-vessels, heavy breathing, sweating and the problem with this is it's impact on your mind/body.

Repeat fight-or-flight fear responses enlarge the amygadala (the part of the brain that starts the fight-and-flight response) it shrinks the hippocampus and stunts development in the prefrontal cortex leading the anxiety, stress, depression and in more extreme cases fears that are embedded can become phobias and in the more extreme develop into psychosis in some.

You don't want your body going through the constant fight-or-flight response during sleep when there is no threat because it's preparing itself for this action when there is no reason too.

Instead you want to have positive dreams so your limbic system is releasing the beneficial hormones of Endorphines, Seritonin, oxytocin and Dopamine. I call those sweet dreams because if you study positive dreaming on the dreaming mind you'll see it can enlarge the hippocampus and help with prefrontal cortex development instead of stunting it like cortisol does.

Lot's of benefits in healthy dreaming practices most are clueless about or misinformed. The science is starting to catch on but it's been so painfully slow.