3 ideas
Economic decisions are made at the margin, comparing the additional benefit of one more unit against its additional cost, not on the total value of a good. This resolves the diamond-water paradox: water is more valuable in total, but because it is abundant, the marginal value of one more glass is low, while a marginal diamond commands a high price.
Prices move toward the point where the quantity buyers want equals the quantity sellers offer; surpluses push prices down and shortages push prices up until they clear. Price controls that fix prices away from equilibrium predictably generate persistent shortages (price ceilings) or surpluses (price floors) rather than the intended outcome.
People respond to the costs and benefits they face, so policies must be judged by the incentives they create rather than the goals their designers announce. Well-intentioned interventions routinely fail or backfire because they alter the payoff structure in ways that redirect behavior toward unintended ends.