So how does one fail in the the pursuit of virtue?
Aristotle notes the existence of another impulse in the human personality contrary to reason and virtue. He writes “there is also observable another element by nature irrational, which struggles and strains against the rational.”
This is what he called passions---emotions and desires.
There is nothing wrong with emotions in themselves. The most meaningful and memorable moments in everyone's lives are such because of the intensity of emotions accompanying those moments. But emotions can be destructive. The worst decisions made often result from basing those decisions on emotions or making the decisions under emotional duress. Aristotle asserts that people must subject their emotions to reason so that they benefit us rather than harm us. This cannot by done through instruction. It is accomplished by training and habituation.
Our desires, too, need the moderation of reason. Aristotle warns that many people “go wrong in enjoying the wrong objects, others enjoying the things with abnormal intensity, or in the wrong way.” These three reasons explain in many cases why virtue fails.
First, Aristotle teaches that the virtuous person must seek the right objects. Everyone chooses what appears to be good. Sometimes, however, what appears to be good is not the real good. The wrong objects simply means choosing the apparent good over the real good. Everyone has made the mistake of desiring something believed to be good only to find out otherwise. One of the most obvious and common modern examples of this mistake is narcotics. People never use narcotics to grow addicted or destroy themselves. They use them because they appear good--say to alter ones mood or anesthetize emotional pain.
Enjoying things with abnormal intensity means enjoying a good to excess. Wine is good until one becomes a drunkard. Food is good until one becomes a glutton.
Sometimes enjoying a good in excess involves making it the supreme good to the exclusion of others. For example, many men and women take the good of work and elevate it to the supreme good, so that they fail to develop other virtues, In addition, family relationships grow weak or one's health suffers. And many young people pursue video gaming with such a passion that they sacrifice friends, school, and work.
According to Aristotle, we must have right desires that follow right reason. In his words, “If the choice is to be a good one, the reasoning must be right and the desire must pursue the same things that the reasoning asserts.”
When emotions and desires follow reason, Aristotle calls them moral virtues.When we fail, they become moral vices.