For Kant, the best way to live is the morally good way. This is not because of morality's bestowal to happiness, but because of the intrinsic value and significance of morality.
The basic principles of morality are principles of practical reason; and these basic principles of practical rationality are 'a priori': our wisdom of these principles is not based on empirical observation or experience. On the contrary, these principles are in some sense 'built in' to the composition of our intellectual capacities. Kant says that the good will is the "condition" of everything else that is good. We can see that, at least in imperfect rational beings like us, acting with a good will means acting from the motive of duty; and the only things that we can will from the motive of duty are the things that duty requires us to will. So, if I am right to suggest that Kant believes that everything that is good is good because it either is the good will or is the object of the good will, then the only good things willed by us are things that are required by duty: acts of helping others, respecting their rights, and so on.
Other things that we want, like fun and amusement, are not directly good at all, though they may be indirectly good if they help us to do things that are directly good.
Kant actually says that the concept of good is unclear. There are things that are absolutely good verses those that are required by duty. On the other hand, there are things that are merely relatively good, that is, good for something or other. Unless the end for which something is useful is itself good or required by duty, then the object is merely good for something or...