We are all striving to be better. There are two ways that we can feel that we have succeeded. The first is to do a lot of hard work to become better people. The second is to ignore the indications that we have faults, which is a lot easier. Hence we spend a significant portion of our lives in denial about ourselves. For a slightly different reason, we deny that there are problems in the world around us. If we acknowledged those problems, we'd feel bad that we didn't do anything to help, and denial is easier.
In Christian culture, denial is deeply rooted. We think that finding fault, in ourselves and in others, is a sin, while if we think only pure, kind thoughts, about ourselves and others, we are closer to God. This raises a number of issues.
First, some people are good at maintaining the illusion of purity and kindness, while they are actually very dirty and very mean, but they get away with it by making the façade more visible than the underlying truth. Bad people are very good at taking advantage of people's eagerness to believe that all is well with the world, and they will violate any trust that we give them.
Second, denial is a disconnect from reality. This gives "good Christians" a reputation for being naïve. But Jesus was not naïve, and here we miss all of the wisdom of the teachings. For example, Jesus loved everyone, and this unconditional love was not naïvely only seeing the good. Jesus saw it all, and still loved us. In our attempt to emulate Jesus, we might only love if we can forget about the bad, so that we can love freely without having to abandon our selfishness. Such was not the lesson at all. Jesus did not tell us to naïvely think that everybody is good, so that we can love and trust everyone. To do God's work, we have to see things as they are, not as we want to see them. When we see something that isn't right, we should fix it with our love. We should not ignore that there is a problem so that we can love selfishly.
It is, in fact, Satan who taught us denial. God created the Universe and everything in it. If we open our eyes, we will see God's work all around us. How is it that we might miss His splendor? Satan, who never created anything, is jealous that God created everything, and that if we simply open our eyes, we will see God's work. Satan cannot change this, but Satan can distort what we see. If Satan can mix up a batch of denial by adding one part of selfishness to another part of laziness, we get disconnected from reality, and can no longer be effective in doing God's work.