OK, so here's the answer from a scientific standpoint. . .I hope it doesn't make you sorry you asked :/. . .
The actual body of a mushroom is underneath the ground. The part that we pick to eat is actually the reproductive structure, similar to another plant's flower. You should be able to pick the reproductive structure without damaging the main part of the mushroom underground, therefore ensuring that the mushroom will send out more, yummy reproductive structures next year. If you dig your mushrooms, however, you run the risk of damaging the main body of the mushroom that is underground. In that case, the mushroom might not survive to produce reproductive structures next year.