In the fast food world, the “limited-time offer” is a remarkably powerful thing. The influx of fish on fast food menus during Lent tends to correspond with an influx of seasonal customers. Last autumn, Starbucks customers seemed ready to riot when news broke that the coffee chain was running out of pumpkin spice lattes, a favorite appearing every fall. The main appeal of the McRib appears to be that it isn’t available all the time, at all McDonald’s locations.

For the most part, fast food limited-time offers, or LTOs, are timed to coincide with a holiday or season. St. Patrick’s Day, March Madness, baseball season, Father’s Day—they’ll all see their share of “special” food and drink items. They’re obvious ploys to juice sales, and more and more of them pop up every year for a simple reason: They work.

The Wall Street Journal recently noted that it’s not merely the “limited time” quality that attracts diners. Sure, we’re motivated by the feeling that we don’t want to miss out on something that won’t be around forever. But there’s often more to it than that. Many LTOs have become traditions that consumers eagerly anticipate with the arrival of every season and holiday, right alongside Halloween decorations and Black Friday sales. It’s almost as if it wouldn’t really be autumn if pumpkin-flavored coffee, pancakes, and shakes failed to appear on chain restaurant menus. About the same time of year that Christmas music takes over every mall, consumers are reliably game to want peppermint and chocolate twists added to almost anything edible.

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Add green food coloring to whatever, and we’ll buy it around St. Patrick’s. OK, so maybe that’s an exaggeration. But there is something almost Pavlovian about the consumer response to LTOs, especially ones that announce the arrival of a season and that trigger fond memories.

In fact, fast food establishments think very carefully about LTOs. They plan menu item rollouts for months, sometimes years, in order to get it just right with consumers. The quick-service industry publication QSR Magazine described “The Art of the LTO” in an article last summer. Gary Occhiogrosso, chief development officer of TruFoods LLC, which owns brands such as Ritters Frozen Custard and Arthur Treacher’s Fish & Chips, explained the thinking that goes into LTOs:

“You need to generate excitement and interest, but you can’t afford to continually extend the menu,” he says. “So that’s where the LTO fits in. It arrives with a product that’s perfect for a particular season at just the right price point, and there’s a sense of immediacy around it. You’ve got to get it before it’s gone.”

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Consumers wouldn’t be worried about missing out on, say, a McRib, if it was always on McDonald’s menus. The sense of immediacy is forced, of course, as is the pressure one feels to take advantage before it’s too late. Regardless, the LTO works its magic on us time and again. It’s not unlike the compulsion so many shoppers feel regarding daily deals from Groupon, LivingSocial, Gilt, and such: The desire to avoid missing out can be as strong as the desire to purchase whatever it is that’s being sold.

What’s next in terms of LTOs? Analysts tell the Wall Street Journal that diners can look forward to a rise in especially unhealthy menu items:

Expect to see more indulgent or high-calorie dishes promoted as limited time offers, says Bonnie Riggs, restaurant analyst at NPD Group Inc., a market research firm. They are generally exempt from the growing number of laws requiring restaurants to list calorie counts on menus. “It’s a good way of getting around that,” she says.

Like all LTOs, these fast food offerings will be short-lived—just like you if you eat too many of them.