What causes Silent Hill to do the things it does?

I've seen MAAAANYYY different theories, several of them revolving around Alessa saying she created the Other World, which doesn't make all to much sense considering she's only in Silent Hill 1 and 3. So what is the real cause for it? What causes it to drag people into the town, change demensions, and torment the characters? I have a good idea of what it is, but I can never seem to put it into words when my friends ask "What's that game about?".