Causes of Head, Neck & Shoulder Pain:
There are two main causes of neck and shoulder pain: structural stress and compression. Virtually all types of conditions and injuries that lead to neck and shoulder pain will have structural stress or compression as main components.

Structural Stress
The most common characteristic of structural stress or poor posture is chronic slouching which causes what is known as a forward head. Slouching forces the head forward in relation to the ribcage, causing the small muscles of the neck to have to constantly hold up the ten pound weight of your head so it doesn’t roll forward and off your shoulders. Imagine how difficult that becomes after a long day at work. Consider this occurring day in and day out for weeks, months and even years... the result is chronic structural stress.
Structural stress can affect how far you can turn your head. To illustrate this, sit in an intentional slouch and turn your head to look to the right and then to the left. Notice how far you can turn in each direction. Now, sit up nice and straight and again turn your head to the left and right. Do you notice a difference in how far you can turn? Most people do. Did you also notice that turning in each direction while sitting up straight is more comfortable? This is because when sitting up straight, all the muscles and bones in the neck and upper back have room to move and do not get in each other’s way.

Compression
The bones of the neck are stacked on top of one another, like a stack of children’s building blocks. The muscles of the neck are like two strong, wide elastic bands on either side of the neck holding the bones in a stack. The discs between the bones form a cushioning and shock absorbing layer that protects the spinal cord and the base of the brain. Discs can be illustrated by imagining marshmallows between the stacked building blocks. If the elastic bands, or muscles, are balanced and in good condition, there is ample space between the bones of the neck to avoid squeezing of the disc spaces. If, on the other hand, the elastic muscles along the sides of the neck are too tight, then the disc spaces are squeezed, squashing the discs (or marshmallows in our example), leading to bulging discs. In severe compression, the walls of the discs break down and lead to herniation of the discs. Pain is the result.
Compression forces the irregular shape of the bones of the neck to come closer to one another. In this state, they tend to get in the way of one another and make it more difficult to turn the head. Often, people will begin to try to unconsciously correct this situation by shifting the head even more forward, or by raising the shoulders to create slack in the muscles at the side of the neck. More pain is the result.