Mark McMinn makes the following observation in his book, Why Sin Matters:

When we see ourselves as "pretty good," we misunderstand the gravity of sin and our desperate need for grace. We place ourselves above others, become their judges, and give them the power to disappoint us.

A physicist friend uses this analogy: Each of us is like a light bulb. One shines with 50 watts of holiness, another has only 25 watts. Maybe the most stellar Christians are 200 watts. But these comparisons become trite in the presence of the sun.

In the face of God, our different levels of piety are puny and meaningless. It makes no sense to compare ourselves with one another because we are all much more alike than we are different.