People choose the alternative which seems best to them because it involves the least cost and greatest benefit. All choices involve cost. Cost is the second best choice people give up when they make their best choice. People respond to incentives.
Incentives are actions or rewards that encourage people to act.
When incentives change, people's behavior changes in predictable ways. Economics systems influence individual choices and incentives. How people cooperate is governed by written and unwritten rules. As rules change, incentives change and behavior changes. Voluntary trade creates wealth.