Why do people say that criticism is an act of gross and insulting ?

Well, I'd say that criticism comes in two forms. The helpful kind that shows someone how to improve in an area and is delivered in a loving and gentle manner. Then you have the kind that is rude and meant more to hurt someones' feelings rather than be helpful. If your' citicism sounds more like the second form then I understand why someone reprimanded you. Criticism should be used to help build someone up. Tearing them down just pisses them off and then you make an enemy.

It depends on the circumstances. For example, did you criticize the person or did you criticize their behavior. If you say to somebody, "I didn't like you telling the boss about my idea, because I wasn't ready and it should have come from me", that is constructive. It tells the person what they did and why it was wrong. But if you had said to them, "you jerk, you always mess me up like this, what a stupid thing that was to do", that is attacking the person. It's not only going to make them feel defensive, but it isn't going to get your point across. Is it fair, is it kind, is it honest, is it true? If it isn't, don't open your mouth!