Indeed, it was probably because man could live on anything and everything that he was able to survive in famines and to get so far ahead of all other sorts of animals.
We still need a great variety of different sorts of food in order to keep our health; so our tendency to become tired of a certain food, after we have had it over and over and over again, for breakfast, dinner, and supper, is a sound and healthy one.

There is no "best food"; nor is there any one food on which we can live and work, as an engine will work all its "life" on one kind of coal, wood, or oil.

No one kind of food contains all the stuffs that our body is made of and needs, in exactly the right proportions.

It takes a dozen or more different kinds of food to supply these, and the body picks out what it wants, and throws away the remainder.
Even the best and most nutritious and digestible single food, like meat, or bread and butter, or sugar, is not sufficient by itself; nor will it do for every meal in the day, or every day in the week.

We must eat other things with it; and we must from time to time change it for something which may even be not quite so nutritious, in order to give our body the opportunity to select from a great variety of foods the particular things which its wonderful instincts and skill can use to build it up and keep it healthy.