The process of searching for employment can be very stressful. You may be transitioning from the exhiliration of graduating from college to entering the job market, or just searching for a position while still attending school. The self-imposed pressure of wanting to secure a stable position can take a toll on your mental and physical health. It is important to be at peak performance as you speak with each employer you meet. You are making a first impression each interview and you do not want to appear tired, depressed, moody or defeated.
There are some very good articles on The Ladder which address the issue.
Dealing with Anxiety