Childhood craving.
Growing up in Indiana, we had a regional chain of fast-food restaurants called B-K Root Beer. They were the old fashioned drive-in style, where you'd park and sit in your car, and the waitstaff would come out to you (I think ours had them on rollerskates, but I don't remember too well). Anyway, I loved going to B-K for their draft root beers, served in a big glass mug. It's a strong summer memory for me, sipping through the foam of a B-K root beer, munching on fries. It may be the greatest soda drink I've ever had.