Governments usually come up with their own definition of the good of the citizens. Some governments believe that people should be protected from harmful products like tobacco. Other governments believe that informed adults are allowed to take some recreational drugs which have been socially acceptable for a long time but not others. Most governments believe that citizens should be taxed the least feasible amount, but they disagree wildly on what is a necessary expense and what is not.
In a democracy, competing definitions of the good of the citizens face each other at the ballot box.
If they are not corrupt to start with, people who get into politics do so because they have strong opinions about the good of the nation and the citizenship. They want to convince the voters of the rightness of their political platform.
Governments also order scientific studies of the impact of their policies. But the way the questions are framed is influenced by their own definition of the good of the citizen. Some might ask "Did the bill reduce homelessness among veterans?" while others might ask "Did the bill increase home ownership among middle class families?"