Under Milton's cosmology, the desire for freedom always moves contrary to the desire for happiness. Satan's rebellion is described as an eruption of pride, but it can also be understood as a demonstration of power (i.e., an expansion of personal freedom). By refusing to submit to God's will, Satan asserts a right to his own identity separate from God. In a sense, Satan's rejection of God's authority is analogous to every human son's rejection of his mortal father, prefigured by the original sin of disobedience. But Satan misjudges the awesome scope of God's power and is banished from heaven.
In Paradise Lost, Milton inhabits a moral universe whose template is the Copernican model of planetary motion. The sun represents God in the center of creation, around which revolve the planets and lesser bodies. The planetary bodies are held in their orbits by gravity, which in our example represents the force of God's authority. Thus, we see that our home, Earth, is not free to travel just anywhere, but moves in a prescribed orbit around its Creator. Milton would say that human happiness is like the limited path of our planet's orbit around the sun. If our planet is ever allowed to veer from that path, darkness and eternal night will follow. It is God's authority that sustains the conditions necessary for both our survival and our happiness. Thus, we see that the dimensions of human freedom move inversely to the potential for human happiness.
This might explain why the Tree of Knowledge appears in Paradise. God's law (i.e., his authority) imposes limits on Adam and Eve's freedom. They are free to do pretty much whatever they want except to eat of the tree of knowledge. Accordingly, the tree of knowledge represents God's sacred law which has been given to Adam. Yet, the law itself creates the possibility for sin. Notice, if there is no law there can be no disobedience, and no opportunity for sin. Thus, the institution of law creates the possibility for crime. And it is only our esteem for Adam's character (his original nature, unspoiled as yet by sin) that allows us to blame Satan for man's fall.
Here, at the very beginning of man's history, we confront the ambiguous character of sin. That man's freedom allows him to disobey God; that Adam's temptation could only result from God's law (i.e., no law, no temptation); that Satan manages to sneak into Paradise despite God's love for man; that man's evolution from a child (a state of grace) to an adult (burdened with sin and guilt) could only occur through banishment from Paradise; that all of man's struggle since the Fall is defined by his desire to return to his original home. Finally, we must ask if a man's ultimate liberation from God is required to achieve his greatest potential as a rational being; and if so, is the cost of such knowledge the loss of Paradise (happiness) forever? Or to put it another way, is the quest for self-fulfillment (enlightenment) worth the cost of eternal happiness? Of course, Plato would say that the apprehension of truth ( philosophy...philo=love + sophia=wisdom ) is the very meaning of happiness. And Milton would agree, but he would phrase things differently. He would say that truth comes only from God, and any prospects for human happiness depend on the love of that truth.