If sales generally feel hard to resist, the sale in front of Aarron Schurevich was the ultimate test: a new Kia Soul just like the one he'd had and loved, at a dealership he trusted, at a moment when he really needed a car. And it was priced $4,000 off, more than a 20% discount.

"I figured that I would be an idiot not to to take advantage of that," says Schurevich, a teacher from Omaha, Neb. "I'd better snatch this opportunity before it evaporates."

After he sped through paperwork and drove the car off the lot, the deal turned sour. Bills arrived with hidden charges. The brand-new car quickly needed repairs. Schurevich now jokes that he paid a tax for being a fool.

"You know it's that kind of voice in the back of my head that's like, 'Are you being a sucker?'" Schurevich says. "And unfortunately, that day, that voice was a little bit quieter than it oughta have been."

Why is it so hard for the human brain to resist a discount? What's the deal with deals? This big-ticket example illustrates all the dynamics that play out when any of us fall for a sale.

How a sale works its way through your brain

When you shop, there's usually a standoff in your brain between what can be described as its emotional and rational parts.

"The human brain has essentially evolved to feel first and think next," says Carolyn Yoon, who studies consumer neuroscience at the University of Michigan.

Spotting something you'd like to to buy activates your brain's reward circuitry. Dopamine-fueled impulses pump you up. Anticipation might have you imagining how great life would be with this new thing if you had it. All this gets especially heightened if it's something you're predisposed to like — say, the same Kia Soul you've enjoyed for years.

The counterbalance is your cognitive mechanism. It might pipe up like a prudent accountant: Do I need this? Is this worth it? How does it fit in my budget?

A sale lands like the thumb that tips your mental scale toward buying.

In fact, the discount itself often registers as a win, delivering its own bolt of joy, says Jorge Barraza, a consumer psychologist at the University of Southern California.

"Not only are we getting the product," he says, "but we're also getting that reward that we discovered something, we've earned this extra thing."

How stores prime and prod us

Stores, of course, know all this and try to push our buttons.

Experts say we often subconsciously believe popular things to be more valuable or more rewarding. Plus, there's our urge to avoid losses — call it loss aversion or simply FOMO, the fear of missing out.

So stores appeal to our crowd mentality: It's Black Friday, and everyone's shopping, buying that thing you'd like. They create urgency: Your favorite car is on sale today only! And they create scarcity: Shop now while supplies last!

"Limited-quantity, limited-time, scarcity-marketing promotions — they get people's blood pumping," says Kelly Goldsmith, who studies this as a marketing professor at Vanderbilt University. "You attribute it to the product: it must be good."

Retailers also try various pricing tricks. For example, picture a store shelf where a medium bag of candy sits next to a larger bag of the same candy.

"How do we make more customers go to the more expensive option? We add a decoy," says Savannah Wei Shi, who researches pricing and decision-making at Santa Clara University.

The decoy is a medium-sized bag. It's much smaller than the other bag, but only slightly cheaper. It makes the big bag look like the best deal, so shoppers buy that one — the most expensive option on the shelf.

Another classic is the suggested price — an amount always higher than the discounted offer, still listed on the tag for comparison. Barraza says people not only perceive expensive things as higher-quality, they actually experience them as higher-quality.

"So the suggested retail price can really pack a wallop," he says. "We can communicate quality to a consumer. But then we can discount it and have consumers think, 'Not only am I getting a quality product, but I'm getting that for a much cheaper price.'"

How to shop smarter

It's really hard to always approach sales rationally — even experts struggle. Barraza says during the last holiday season, he almost bought a video-game system simply because it was on sale.

Deals and sales certainly can be good and useful. And Barraza underscores that prices are subjective, so a discount may be unattractive to one person but appealing to another.

One buying strategy experts recommend is to make a shopping list in advance and then, stick to it. Another is to research items — beforehand or on the spot, checking online — to weigh whether the sale really is a good deal.

The main thing is give yourself time to cool off from your instant reaction.

"The ability to think can override the emotional state," says Yoon. "The more you spend time thinking and bring your cognitive processes to bear ... you have a shot at basically saying, 'No, I think I'm going to pass,' even though that wasn't your first inclination."

In fact, this is what stopped Barraza from buying that gaming system: Standing behind about 20 people in line to check out, he had time to ponder whether he actually wanted the thing or was simply swept up in the excitement of a sale.

"I was saved by that line," he says. "A little bit of time can go a long way."

Remember: we feel first and think later. Your internal accountant just needs a moment.

NPR's Joe Hernandez contributed to this report.