Aristotle considered the irrational side to be lower because it was what we had in common with animals. Evidence for this division of the soul came from the fact that emotions often times make one act differently than a rational decision.
What is a good life? Aristotle believed the goal of human life was eudemonia, or happiness. As a complex notion, however, eudemonia combines self-sufficiency, health and pleasure with success, friendship, and wealth. Happiness is, on the one hand an emotional response, and, on the other hand, a product of reason. Virtue and excellence in life come from the combination of the irrational and rational parts of the soul. Most people seem to live for the pursuit of pleasure. The good life is a life of pleasure. However, for Aristotle, pleasure is good only because it motivates one to act virtuously. Excellent activity is inherently pleasurable to the virtuous person. In other words, the action causes pleasure, as opposed to the emotion, or when the activity of the lower part of the soul is in accordance with reason. It seems to follow from Aristotle that in order for the good life to be pleasurable, one must be able to feel pleasure from actions that are also virtuous.
What distinguishes the Epicurian view of Lucretius from that of Aristotle is that Lucretius did not think this habituation of emotions was necessary. In considering what characterizes human existence, Lucretius appealed to reason, and only reason, because he believed that emotions, being irrational and animalistic, blocked the rational, humanistic, capacity to operate at the fullest extent. Lucretius also believed that the feeling of angst, inherent in the Aristotelian view, was the fear of death, and that gradually this emotional response would become incapacitating. This fear came from the irrational, emotional response to an afterlife, and the degree to which it would be pleasurable. Lucretius used the rational Epicurian ideas as a kind of therapy from this angst, and a way for human reason to stamp out the effects of the emotions.