People demand different goods and services for all sorts of different reasons.
The economist's strategy is to abstract from all that. That's just what the utility approach does, and it shows that we can say important things without worrying about all the reasons why people buy things. That's pretty important, abstract as it is. As we'll see, that has many more applications in microeconomics, and a few in macroeconomics as well. And it gets us started in the practical business of cost-benefit analysis.