Why should we drink soup first !

Cultural. But according to one study I read, we should eat the most easily digestible food in a meal first and then progressively the more concentrated food, ending w. the food that takes the blongest to digest. That, however, seems like a chemist would have to analyze most meals.
BTW, most cultures eat all their salad first, but some eat salad along w. the main meal, and some cultures eat it last.