I was taught this originally for backpacking but it really always makes sense once you think about it. Pouring onto a hand keeps dirty hands out of the rest of the snacks in much the same way as a drinking fountain manages to be hygienic by shooting out the water.
Some alternative sides to sharing: compliments are gifts and give gifts others can give. And two more about germs: sneezes and coughs, and the Dracula sneeze.
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