usually mine are the result of stress, conscious or more usually subconscious. i dream pretty symbolically- i can usually pick apart their meanings later on and realize what inspired or spurned the dream. when i have a bout of nightmares, they usually point to my being stressed about something that i then have to introspect and figure out what it is- school, family, personal failures, social anxiety, love frustration, etc.
often, i'm a lucid dreamer, so when i loose control of a dream and feel lost to its disorder, its usually a clear sign that my mind itself is addled up over something. in these cases, i think that nightmares are the mind's way of manifesting/notifying one of mental disturbance.
sometimes though, i believe they can be the result of OTHER'S negativity, directed maliciously at one, either in the form of a subconscious mental attack/projection of anger/hate, or as a rarer, more formal curse or hex. this problem only really applies to those of us who are 'in' with communities or groups of witches or magicians, but you never know who you're pissing off at work. it can get ugly. similarly, i think unwanted negative spiritual attention can cause nightmares (demons or spirits)- i, personally, can usually tell the difference.... they feel off, and not like the nightmares caused by stress, because they're just sick and twisted and violent, with no real purpose or source. these are the dreams that leave you sweating and disturbed when you wake up- they're very rare, but they're decidedly devoid of meaning when you try to interpret them- they just are. i've found that a basic exorcism or banishment of such forces clears the problem up immediately, even after a lengthy bout of them. most cultures actually held this belief or still do- dream catchers are a great example of one believed way of keeping spirits from tampering with dreams.