No entity (except perhaps a major deity) can know what another individual wants. Some guesses could be made - based on empathy (I feel this, therefore he must feel it too) or previous observation (she always asked for two sugars before, so I think that's what she wants), but they are not reliable. Social scientists, social workers and governments are notorious for believing they know what is good for individuals better than they do.
If an individual is rational, then it will make the best decisions for itself. So the best measure of what is best for the individual is what it would choose to do. But the only way to test if it would actually do what you think is to let it make the decision, and observe it. This will never give an absolute measure of utility, but it will give a relative measure between the choices it was given - through expressed preference.
You cannot make a people happy by deciding what is good for them, and providing it, because you cannot know what each of the individuals want. You can only provide freedom, and let them make their own decisions.
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