When choosing a meal, the brain does some crazy things. There is a lot more that goes into a food decision than immediately seems apparent.
According to a recent article in the New York Times, consumers are more likely to choose unhealthy items at fast food restaurants if the shop has healthy options on the menu. It seems unlikely, but the claim is that it's due to "'vicarious goal fulfillment.' By seeing a healthy menu option at a restaurant, 'it basically satisfies that goal to be healthy,'...and gives consumers leeway to order what they want."
Additionally, humans can actually train their brain to crave or desire healthier options. Much like muscle memory, several consistent exposures to healthy food can turn a donut-craving crazy into a kale fiend.
So, the next time you step into a haven of junk, make the conscious decision to order the healthy option. Do that enough times, and it will start to become second nature!