How does the monopoly's deadweight loss affect market surplus and the economic pie?
Consider the following two examples:

There is a consumer who is willing to pay 50 dollars for a good. (Reservation price is 50.) There is a seller who is willing to sell for 30 dollars. (Marginal cost is constant 30.) If they make a deal at any price $p$ the total surplus they enjoy will be $(50 - p) + (p - 30) = 20.$ The individual surpluses need only be non-negative because otherwise one party would probably not consent to the deal. The price that is finally reached depends on their bargaining power. With a monopoly all the bargaining power is on the seller's side, so the price would be $p = 50$. Total surplus is still 20. So there's no loss.

However if there are two consumers, one with a reservation price of 50 and the other with 31 then at price 50 only one of them (the former) will buy the good. At price 30, both of them would buy the good and total surplus would be $(50 - 30) + (31 - 30) + (30 - 30) = 21$. But the monopoly's profit is 20 dollars if it sets $p = 50$, and 0 dollars if it sets $p = 30$. So it will choose the former. Because of uniform pricing, a good that would increase the total surplus is unsold, creating deadweight loss.