<< Why does the world revolve around money? >>
The most obvious answer is because all of the essentials of life - food, clothing, shelter, etc. - cost money. So in order to acquire them, you obviously need money. For your example of a "comfortable middle class lifestyle" these days, you would need to acquire a not-inconsiderable amount of money to be able to afford the essentials of supporting such a lifestyle. Depending on your demographics, etc., that could equate to maybe $60-120,000 (US) a year. For example, here in Ohio, you could lead a very comfortable "middle class lifestyle" on $70-75K a year. In California or NYC, it's probably on the high end (?) of my scale listed.