Why would the government know better than a particular person what is in that particular person's best interest for them to do?
Because they have more information about the global picture, a longer-term view, lack that person's bias and can consult expert opinion. How many of these are actually followed would be the difference between good and bad government IMO. And if you've read about behavioural economics, then you know that people don't act in their own self-interest a lot of the time due to how our brains work and evaluate things like sunk costs, risk vs. reward or relative value.
Also, asking about a particular person's best-interest is a straw man, government is about society as a whole, and because society is made of up of lots of people with all kinds of complex interconnections, often what is best for one particular person isn't what's best for everyone as a whole. Being a criminal might be best for one person, but negative for lots of other people - and you can't just have criminals, or they have nothing to steal.