Obsession
The premise, or whatever this movie is about, is so simple, it's about an OBSESSION.
But how did it become so great with such a simple idea? Because of the EXECUTION.
The budget of this movie was also just around 750K U.S. Dollars, which is categorized as a low-budget movie.
And how did they maximize it? With the ACTING, the MUSIC SCORING, with the uneasy feeling that most horror movies don't have, and lastly, the unsafe feeling throughout the movie.
We are scared because of things that we don't really understand. We worry about what could happen because of the UNKNOWN.
And for me personally, that is what HORROR is