Stress; not good for humans, not good for dogs or anything for that matter.
The below definition was taken from Wikipedia.
Stress is a biological term for the consequences of the failure of a human or animal to respond appropriately to emotional or physical threats to the organism, whether actual or imagined. It includes a state of alarm and adrenaline production, short-term resistance as a coping mechanism, and exhaustion. Common stress symptoms include irritability, muscular tension, inability to concentrate and a variety of physical reactions, such as headaches and elevated heart rate.
So what stresses out a dog? Anything and everything can stress a dog; just like in humans, they are all different. Dogs all react to external stimuli very differently so what may stress out the dog next door is probably entirely different from what stresses your pooch out. With regards to training and modifying behavior; it is of extreme importance to watch a dogs reaction. The slightest bad reaction can snowball into something very large requiring more modification.
Some dogs are much more prone to stress than others. My two females are not the type to stress; my boy on the other hand is a stress monkey. It doesn't take much to have him worried and stressing. Stress can be caused by something fearful; confusion, excitement and even anticipation. Even really great situations or animate objects can cause stress. If your dog is triggered into bouts of over excitement they can easily cross a functioning/nonfunctioning line. Again Luke can cross that line easily so I watch his excitement level alot.
I've heard guardians often telling their dog to stop being silly when they show fear towards some object. It may be something like a troll on someones front lawn. But for some reason your dog does not understand that it is simply a plastic lawn ornament and is coming unglued the closer you get to it. Fear is the cause of the behavior; but the behavior is a stressful reaction to the fear. Stress is what happens to the body when we or your dog are not equipped to deal with an external stimuli.
Often a dog will exhibit a fearful response to a normal daily activity. You may have no idea what caused this odd fear and you may never know. The important thing is that you deal with the reaction, no matter what the cause is or was. It is nice to understand why your dog may be afraid of a certain cupboard in the kitchen or your ice dispencer but if you don't know why you can still help them get over it which then gets rid of the stress related response to the initiating factor.
Counter conditioning is used to create a different response when stress is the default application. Let's say your dog is afraid of the ice dispenser (I actually had a case like this,) your job is to create a positive association to the ice dispenser. Creating an entirely different association to a fearful trigger takes alot of patience and baby steps. You must look at the smallest of achievements as success; never moving on further until each step is achieved.
Stress is a very real internal response and it can be mild to extremely dangerous. Learning to see the signs in your dog takes time and takes close visual supervision. Just recently we were out at a park when a young child approached. Both dogs were fine at the approach but as the child came closer Tilley got uncomfortable; the child came up to her at the side and Tilley pulled away. But before the child was even 5 feet away from my dogs I was down and right there. I know toddlers and toddlers like to hug; which my dogs do not like to do with strangers. So as Tilley started ever so slightly to lean away I put my hand out stopping the child. The child was so involved with all the dogs around she didn't even realize I had moved her away, neither did Mom unfortunately.
**As a side note it is never a good idea to let children or anyone hug your dog. Even if you think it is a good idea and that your dog will be fine; a bite to the face cannot be undone.
No matter what the cause to a stressful reaction is; you must work to help your dog learn to deal with it. Typically stress grows on it's own but it doesn't go away on it's own. No body or dog wants to live day to day with stress; it is no way to live. So figuring out what is causing the stress is the first step; then working at creating another entirely different mental reaction to the trigger is the next step until it is gone.
One step at a time.