They are constantly bickering, constantly arguing, constantly not trusting each other, and yet they will die for one another at the drop of a hat. Several times, mind you. Supernatural is addictive because it barges its way into the watchers' hearts and then proceeds to detonate like an atom bomb.
Not only does Supernatural speak to the people who love to see monsters and the brothers hunting and killing demons, but it also speaks to people who love classic rock music. From the first episode, Dean Winchester loves his hard rock music, much to the chagrin of Sam. Of course, fans love it, too.