There are many reasons that people would keep the truth from someone. Not all of them are good, but sometimes it's better for your personal and professional relationships to refrain from the whole truth. Some only serve to lessen your own conscience, and not anything that would help the other person. Some are helpful to the person learning the news. And some serve only to manipulate.
With compulsion comes a lack of control, and that lack of control can lead to stress. Remember that we have a staff of mental health personnel to help you through this.