Judgment is the mental ability to understand something, form an opinion and reach a decision. A decision is a judgment, unless you let a coin toss decide for you.

judgment--Mental act of evaluation through comparison or contrast. The spectrum of biases, isms, prejudices, and stereotypes all contribute to this metaphorical "judgmental witch." 

The answer to why we judge  is very simple, because we know only the partial truth about the issue. At times that is also unknown. On such assumptions, we base our strong opinions and on these baseless grounds make our judgments.

We fall to such a stoop, that we often ignore our inner voice which is practically screaming at that time. On such flimsy foundations, we build our judgments, often neglecting the inner voice that urges restraint.