Jesus Christ said: "Do not judge others, and you will not be judged. For you will be treated as you treat others. The standard you use in judging is the standard by which you will be judged." (Matthew 7:1-2, NLT)
Why? Why shouldn't we judge each other? Aren't we capable of making observations, reasoning and formulating conclusions?
Many people would say that it's not our place to judge each other (which is certainly true). Others would point out that God has given the authority to judge our behavior to Jesus Christ. (John 5:22, 27) However, although both responses would be technically correct, they don't get to the reason why God hasn't entrusted us with judging each other.
The real reason that God doesn't want us to judge each other is because our judgment is impaired. After all, our judgment is limited by what we take in through our five senses. In other words, we can only judge what we see, hear, taste, smell and touch. Moreover, our judgment is influenced by our personal intellectual abilities, education, experiences, prejudices and biases.
When God sent Samuel to Bethlehem to anoint one of Jesse's sons as king, the wise old prophet had a hard time discerning which one God had chosen. At first, he thought it would be Eliab because of his stature and appearance. (I Samuel 16:1-6) Nevertheless, God told him: "Don't judge by his appearance or height, for I have rejected him. The Lord doesn't see things the way you see them. People judge by outward appearance, but the Lord looks at the heart." (I Samuel 16:7, NLT)
We can't know what makes a person tick. We can't see what's going on inside of another person's head. Put another way, we don't have all of the facts at our disposal. As a consequence, our judgment of each other is always flawed.
My grandfather was a good case in point. He was in the U.S. Navy during WWII, and he "cussed like a sailor." Nevertheless, he was a kind, good and decent man. Even so, based on outward appearances, one might judge him as being a crude heathen.
Likewise, most of us have known at least one or two little silver or purple-haired old ladies who attend church every Sunday and wouldn't dream of saying "fart" out loud, but are as cold as a freezer on the inside. Who is the better person when we have all of the facts at our disposal?
God doesn't want us judging each other, because we can't see what's going on in the other guy's (or gal's) heart!