Everyone has it to some degree. Some people claim to never have it, while for others, it can be debilitating. But nervousness is often a very important participant in a photo shoot, for good or bad. Photo shoot subjects, unless they are professional actors or models, can come to a shoot bringing a suitcase for of nervousness; baggage in both senses of the word. People generally bring their nervousness to a shoot because they are unsure of what will happen, often feeling as though they will be responsible for knowing what to do.
As I have mentioned on a couple of occasions, I was a musician in a former life, and so thinking about nervousness has always been a part of my life. When you perform, whether it be musically, verbally, or some other way, you might have feelings of doubts or inadequacy, which manifest themselves as nervousness. I know that when I performed, especially when classical music was involved, I brought along with me the years of music teachers who had criticized my "sound" and technique. I still have the New Hampshire "All State" evaluation form from one of my auditions in high school which labeled my tone as "harsh". Yum! So in my typically defiant way, I went on to study the trumpet at Berklee, always putting myself into situations where my sound was of primary importance, eventually developing it to the point where it was the best aspect of my playing. But if you aren't aware of it, when you stand in front of an audience to perform, those old criticisms can come swirling back. Never mind that your audience has no idea that you have brought such baggage.
Getting back to the subject in a photo shoot though, it is often the case that the nervous person that arrives at your studio or location to be photographed has brought years of such baggage, but in this case, about themselves; their appearance, their behavior, etc. The absolute worst thing that a photographer can do is to have his or her own baggage on display as well. Your subject has to feel that you are in charge of the situation. You need to make the person feel that they will be told, every step of the way, what to do, where to stand or sit, how to do so, etc., and that you will be sensitive to their discomfort and will be certain to alleviate it.
That being said, there is a certain kind of nervousness that is good for the photographer to have. It's really important to leave the destructive, baggage kind of the nervousness at home. It won't help you, the photographer, or your subject to feel as though you are in control and know what you are doing. It will signal to your subject that their own feelings of nervousness are validated. When I doubt myself, I use the recommendation of the excellent photographer, Zack Arias, that you just need to show up at every shoot as though you are shooting for Rolling Stone or some similar gig. As I said, I used to get really nervous as a musician, when performing classical music because of my feelings of unworthiness, but I found it helpful to step out of myself and imagine that I was Maurice Andre. I think that experience has helped me a great deal as a photographer to do likewise. It's really important to realize that there is a positive kind of nervousness though that need not adversely affect your abilities to shoot well, nor the ability of your subject to be themselves, but will, in fact, enhance your performance. That "edge", which I always feel before a shoot, and don't suppress, helps me think in that same out-of-body way. Rather than getting bogged down in what to do and how to do it, I start to think as I imagine Arnold Newman, Richard Avedon, or even Joe McNally would be thinking in that same situation. The difference that I can see in the faces and the demeanor of clients when I calmly control the situation on a shoot is pretty amazing. If you are a photographer, use this edge! If you are looking for a photographer, I would urge you to find one who has it.