Nothing lasts forever. Your favorite restaurant closes down without a word. That new hit song gets overplayed on the radio and forces you to find a new favorite. The world of beer abides by the same forces: Eventually, a beer will see its last day on a draft line or in your fridge.

And yet it’s nearly always surprising. How can a beer that you’ve grown up with and loved suddenly see an unexpected demise? And, more importantly, what happens after?

Let’s peer into the glass and determine why flagship beers — beers that often make up more than 50 percent of a brewery’s revenue — are disappearing.

First, brewers say, the consumer’s mentality of choosing one brand over all other brands is no longer as strong as it once was. Brand loyalty is weakening. People who go to venues like HopCat, Mr. Brews or World of Beer are faced with 150+ different beers to choose from, and breweries expect them to pick the one they’ve always gotten? Fat (Tire) chance.

The number of brands to which one can choose to be loyal isn’t helping the life span of flagships either. With more than 4,500 breweries nationwide (more than 100 breweries in Wisconsin alone), the competition for brand loyalty is fierce. Beer drinkers are constantly asked to try something new, something shiny, something different, making it difficult for a brewery to craft a flagship.

It goes without saying that flagships are defined by their quality. There’s not enough ad money in the world to counter a poorly brewed beer, according to Deb Carey, president of New Glarus Brewing Company, best known for Spotted Cow. “Some declare a flagship and put up posters and market it that way, but that’s just not the way to go about it.”

Otto Dilba of Ale Asylum is a firm believer that no brewery can dictate what its flagship is going to be. “We had an inkling, a hunch, and that took off the most, but it’s really consumer-driven,” Dilba said. “If you’re just starting out then you put your brand out there and hope the consumer enjoys what you’re doing. That’s it.”

While it’s usually the allure of new brews that sinks flagship beers, Carey believes that there comes a time when every experimenting beer drinker gets tired of all the new brews. “Consumers have so many choices that they are overwhelmed or become tired of trying different things — they get brand fatigue,” she said. “What you do is you just hang in there and hope that someone will notice that you’re something worth purchasing again and again.”

But the challenge to flagship beers comes from within the brewery as well. Brewers themselves enjoy the challenge to make a beer better, to push the envelope, to see what they’re capable of making — and, in some cases, remaking — in hopes of capturing the ever-changing palate of the beer drinker.

The irony is that, for many breweries, having a successful flagship is what provides them the opportunity to experiment with other beer styles. “A brewery’s flagship beer is often the safety net for a brewery,” Dilba said.

But which beer ends up being the brewery’s flagship can change over time, as New Glarus is finding out.

“We tried a couple different beers before Spotted Cow took off — which at this point is our flagship beer,” Carey said. “But it might not be for much longer. We’re seeing Moon Man becoming more popular.

“We thought lagers were losing their identity and we thought there was a great market for that, so we took our first shot at a flagship beer with Edel Pils,” she said. “But you can’t argue with consumer preference.”

Flagships are typically India pale ales, ambers, pilsners or other common styles that don’t require much capital or time on the brewer’s part to make.

But there are exceptions. Central Waters Brewing has hit success with its bourbon barrel-aging program, and is on its way to being one of Wisconsin’s first breweries to have a bourbon barrel aged flagship beer, Brewers Reserve.

Even established breweries like Ale Asylum are seeing a shift in their flagship. While Hopalicious is the moneymaker, Dilba said, “We won’t be surprised when Velveteen Habit takes over.”

The smartest move an upstarting brewery can do is to look at what’s trending in the coasts, according to Dilba. “When we first started 10 years ago, there were not many IPAs and truly hop forward beers. We knew there would be a need for Hopalicious.”

What’s special about the changing landscape for flagships is that while one flagship beer might die, it rarely stays dead forever.

Take Stone Brewing Company , which in recent years discontinued its flagship Stone Pale Ale only to turn the recipe over to homebrewers who could then replicate it on a smaller (typically five-gallon) system. And then there’s Old Style and Schlitz, two beer brands thought to be forever in the history books, but are now being reintroduced (Old Style being so in its hometown of La Crosse).

So if you’re not ready to let go of a flagship beer, there’s a reason they say beer brands never die. They just cool down in the cellar for a while.