In Tanya, R. Schneur Zalman points out that on one hand, man is prone to selfishness and self-justification. On the other hand, man is in control over his impulses; he is not an animal and has the free will to act as he wills at any given time.
In other words, we might not be perfect, but we have the choice to do perfect. Or to put it in psychological terms, not everything that is wrong with us on the inside do we necessarily have to bring into expression on the outside.
This is the perfection which, R. Schneur Zalman tells us, we can achieve—to become a person who, despite being rife with imperfections on the inside, chooses to behave perfectly on the outside.
Feeling like doing something selfish and rotten but forcing ourselves to do something altruistic and noble is called a decent human being. Whenever we overcome our impulses to behave in a particular way, we are making the decision to do what ought to be done. Behavior is a choice.
It's like asking a Jew, “Did you eat on Yom Kippur?” and he answers, “Well, I felt hungry in my stomach.”
He did not eat! He didn’t. He just felt like eating..
Our job is to put our own imperfections aside and take actions that help make a perfect world.
A Jew has not only the license but the obligation to pursue perfection in his or her deeds. After all, there is really nothing stopping us. “Everyone is just as much of a mentch as he wants to be.”