It’s winter again, so on top of having to go back to class after excessive eggnog and roasted ham, all of your favorite television series have ended or are starting again soon.

With some shows that haven’t been on air for too long, like HBO’s “Bored to Death,” it seems like a cruel joke to make us wait to continue the chemically altered adventures of Jason Schwartzman and Ted Danson’s characters. But for other shows, like Showtime’s critically acclaimed series, “Dexter,” it may be time to put away the bloody steak knife, for good.

Don’t take that the wrong way, I love “Dexter.” The characters are pleasantly twisted and Michael C. Hall is an amazing actor, but the story has just gone flat, lost its direction and begun relying on an obvious formula, in this case introducing celebrity guest Julia Stiles for the season as a character meant to make Dexter feel the emotions he supposedly doesn’t have.

You can see formulas like this in almost any series that goes beyond three seasons. “X-Files” was always Fox Mulder’s weird antics and theories versus the persistently ignorant, but ever-endearing Dana Skully. “Scrubs” was always Zach Braff’s goofy character, J.D., being obnoxiously naïve opposed to his narcissistic yet loveable mentor Dr. Cox.

The problem with great TV shows is, unlike in the U.K. where it is rare for a show to go longer than two seasons, Americans are always demanding more, and the series lose what was initially appealing about them, leaving only a recipe loosely based on what worked in the initial success of the show.

For example: NBC’s series “The Office.” In England, where the original premiered with Ricky Gervais, the show lasted 14 episodes. Because, as much as I enjoy watching social invalids trying to work together in a small office, there’s really only so much you can do with it before it becomes excessive.

And Steve Carell is leaving the show anyway, so what’s left? Remember what happened to “That 70’s Show” after Eric left? Nothing, because there was no reason to watch.

The problem with letting a show drag on until the plot gets old or the main characters leave is that it cheapens the genuine experience of watching a good show within its first few seasons, before it leaves the realm of “art” and becomes a marketing ploy.