Situation 1) You drive up to pay for your food, you pay the price you were expecting, you leave with the same mood you arrived with.
Situation 2) You drive up to pay for your food, you find the person ahead has paid for you, you pay nothing, you leave happier because someone has been kind to you and you get a free lunch.
Situation 3) You drive up to pay for your food, and you find the person ahead has paid for you. You feel happier because someone has been kind to you, and in turn you pay for the person behind you. You pay an unexpected amount, possibly more than your own food cost. Now you leave with a lunch you may as well have paid for, but you leave happier.
In situation 2, the free lunch looks like the thing that makes you happier.
In situation 3, you might pay more than your lunch price and yet still leave happier.
In situation 3, everyone in the chain has gone to get some lunch, got some lunch, paid for some lunch ... and left happier.
And yet, if you tried to set it up to work this way - either the drive thru had a rule where you always paid for the person behind you (and if nobody was there you paid a random amount, or the average price of the previous N orders), or if there was a social expectation of same (i.e. like tipping, unwritten but obligatory), then I don't think it would work. In fact, going there expecting to pay an unknown amount seems like it would make me feel worse; and going there expecting a 'free' meal would tend to make me either order much more (it's free) or feel obliged to order less (someone else is paying), but either way be less comfortable about the whole deal.
It's not surprising to me that in the small scale, receiving kindness and/or doing something kind feels good. But it is surprising that on a wider scale, one system where everyone orders/pays/eats has minimal effect on mood and another where everyone orders/pays/eats results in a chain of tens of people feeling happier. (Side questions: It's easy for the article to present this as a chain of happiness, but how many people in the chains of tens of cars didn't feel happier, and felt indifferent? And how many felt worse yet obliged to participate to avoid looking stingy?)
My guess is that if it expands to be a social norm, it will hit the same kind of backlash as tipping does - arguments about whether taking your food and not paying for the people behind makes you a horrible person, or whether it's okay to pay for just the price of your meal for the people behind / whether you should be willing to pay 10% or 20% more than your meal, or whether if you're paying a random price because there's no car behind you, is it okay for the restaurant to come to depend on pocketing the difference for its profits.
Could we systematise the kindness/happiness effect without ruining it?