Why am I always thinking up depressing stories in my head?

I am a naturally a very happy person and have a balanced life. However, I often find that every now and then I form stories in my head. I am also a dramatic person and am quite good at English especially short story writing assessment. Lately, however, I have found that the stories that I come up with in my head involve very intense problems eg. Abduction, murder etc. They are never involving myself or other people and (if I may say so) are quite interesting story lines. Is this worrying or am I just a dramatic person with an overactive imagination?