In 2015, flyers filed 15,260 complaints with the Department of Transportation. That was a 34 percent increase over 2014’s 11,365 complaints.

The more damning statistic, however, is only visible in the longer view: 2015 was the worst year for traveler complaints since 2000, when flyers filed 20,564 gripes with the Department.

Helpfully, the DOT’s Air Travel Consumer Reports provide rankings of the airlines’ performance, based on the number of complaints received for every 100,000 passengers served. Here’s the list for 2015, from best to worst:

1. Alaska Airlines – 0.50
2. Southwest – 0.52
3. ExpressJet – 0.62
4. SkyWest – 0.65
5. Delta – 0.74
6. JetBlue – 0.86
7. Hawaiian – 1.06
8. Envoy – 1.45
9. Virgin America – 1.66
10. United – 2.85
11. American – 3.36
12. Frontier – 7.86
13. Spirit – 11.73

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It’s no surprise that Spirit and Frontier were the most complained-about, by far, or that Alaska and Southwest topped the list. Virgin America’s mid-pack ranking might raise some eyebrows, though, given that airline’s history of winning service-related awards. (It was ranked first on the recently published Airline Quality Rating report, for example.)

More surprising—and a mystery the airlines themselves would do well to unravel and address—is the disconnect between the complaints and the airlines’ quantifiable performance. Because even as flyers’ satisfaction tanked, the airlines were upping their game, registering improvements in delayed flights, mishandled bags, and denied boarding.

So why aren’t flyers happier?

My theory is that the complaints, while nominally triggered by service or operations lapses, are actually a reflection of travelers’ generalized dissatisfaction with the cramped seating, long lines, and consumer-unfriendly pricing and policies that pervade air travel today. So while airlines may be doing a better job of getting their customers from point A to point B and losing fewer bags, the seat-of-the-pants experience is more unpleasant and stressful than ever.

That’s a reliable recipe for unhappy customers. And unhappy customers complain.

Reader Reality Check

What’s behind the rising tide of air travelers’ dissatisfaction?

After 20 years working in the travel industry, and almost that long writing about it, Tim Winship knows a thing or two about travel. Follow him on Twitter @twinship.

This article first appeared on SmarterTravel.com, where Tim is Editor-at-Large.