If you want to understand why “emotional support animals” on airplanes have become such a flash point, consider a striking seasonal statistic. This Valentine’s Day, the National Retail Federation projects, about 21 percent of Americans bought a present for a pet, spending a total of $751 million.

The debate over animals on airplanes is part of a bigger cultural shift overturning existing norms about when and where pets are appropriate.

“The humanization of pets continues to be a driving factor for the pet industry,” reports a study by the American Pet Products Association. Pet owners born between 1980 and 1994 — aka millennials — are leading the way. They’re feeding pets organic foods, taking them to day care instead of leaving them home alone, buying them health insurance, paying extra for flavored medications, throwing them parties. “Pets have come a long way in the past couple of decades, going from being outside dogs to sleeping in our beds and having their own Instagram accounts,” New York veterinary technician Natasha Feduik says.

Pet owners increasingly treat their animals as full-fledged members of the family and extensions of themselves — and expect everyone else to treat them that way as well. “I emotionally see myself as a ‘mom’ to my fur babies,” Feduik writes, speaking for many. “I have two dogs, a cat and three birds, and they are my world.”

If the soaring number of animals on planes represents “a fascinating case study of how mass cheating can become acceptable,” as David Leonhardt of the New York Times argues, it’s also a prime example of the humanization of pets. After all, you wouldn’t put your children in the baggage compartment.

Besides, it’s not as though pet owners without serious medical issues are exactly lying when they claim they need their fur kiddies for emotional support. Flying is stressful even to those without outright phobias and, barring the occasional hunting dog, emotional support is the whole point of having a pet. The animal is there to be adorable and make its owner feel loved — to provide comfort and joy amid the strains of daily life. So it’s easy to rationalize your online purchase of an Emotional Support Animal vest, and even to justify your furry friend as a public good. The other passengers will light up to see your adorable pooch! Only meanies don’t love animals!

And here’s where the breakdown of existing norms starts to bite.

Until recently, the norm was for people who disliked, feared or were allergic to animals to tolerate brief interactions on the street or in a pet owner’s home. They understood that theirs was a minority view. But they could also count on limited animal encounters. Offices, restaurants, hotels — not to mention airplanes — were pet-free zones.

No longer. Pet evangelists have been gaining ground, especially in making workplaces dog-friendly. “Dogs — with their nonjudgmental, unconditional love, team spirit, sense of humor and the ability to lower blood pressure among ‘co-workers’ — can immediately transform any workplace into an ‘executive retreat,’ ” animal trainer Bashkir Dibra writes in the introduction to “K9-5: New York Dogs at Work,” a 2015 book featuring portraits of dogs at workplaces including “The Daily Show With Jon Stewart” and Martha Stewart Living Omnimedia.

About 11 percent of U.S. pet owners now work in places that allow animals, compared with 8 percent in 2014, according to the American Pet Products Association’s most recent National Pet Owners Survey. Pet-friendly workplaces include such high-profile companies as Alphabet Inc. — whose official code of conduct declares that “Google’s affection for our canine friends is an integral facet of our corporate culture. We like cats, but we’re a dog company” — and Amazon.com Inc., whose campus includes a dog-level water fountain next to every one for humans. “Amazon’s dog-friendly environment helps reduce stress for all its employees,” declares Seattle DogSpot, which named it the city’s most dog-friendly business.

That’s not true, of course, unless the business hires only dog lovers. For some people, dogs increase stress.

“We live in such a dog-adoring culture that it’s hard to admit when you aren’t totally enamored of them. What you are supposed to feel — what you must always feel — is love,” writes former Amazon employee Corina Zappia. As the company planned its move to a fancy new complex, Zappia, who had a traumatic canine encounter as a child, hoped for an office on a dog-free floor. “I would like to be on a dog-free floor,” she confessed in an e-mail to her department head, “if that’s OK.” It wasn’t.

Even with a note from her allergist, Zappia had to settle for sharing a windowless dog-free office on a dog-populated floor. One employee regularly walked his dog around the office, while others kept their office doors shut so their dogs couldn’t escape.

The conflict isn’t just a matter of clashing tastes. Despite what you may have read online, there aren’t any reliable figures for how many people are allergic to dogs and cats, but 10 percent seems a decent estimate. For most, the allergy means a runny nose or itchy eyes, but in some cases, the reaction can be life-threatening. Yet even then, pet lovers have trouble empathizing. “People that are dog lovers, they go bananas over this,” says a friend with serious allergies. “They’re really willing to go to the mat” to keep bringing their dogs to work.

So a new culture war is brewing, not over race or sex or religion this time but over pets in public places. Will we welcome them? Must we? And what happens to the minority who object? Are they destined to be ostracized and unemployable? Or will we again sort ourselves into tribes, with some of the country’s most desirable jobs marked Dog Lovers Only? The fights over emotional support animals in the air could be the start of a long and nasty struggle.