Western TV shows need to have proper endings. Almost all of them are "Can we keep this going for another season and still make money?". If the show was good to start with, it will have enough fans to support it, despite the decline in quality and the horrid ways they use to keep it going. It's one of the things anime does right most of the time: they aim to tell a story, and after one or two seasons, the story will come to a satisfying conclusion.
Dexter is almost an exception to the rule, as the story for each season does actually last one season. It's well worth watching. Just like House, which has no real story and thus House can continue being awesome for as long as he likes.
