Try this out.. walk up to a colleague/friend/family member who is standing or sitting in a conversation or working at their desk. Say "Hi," put your hand on his/her shoulder and say "Relax." Most likely their shoulders will drop on both sides.
Most of us have, will have, or have had neck and upper back tightness at some point. Stress, posture, activity, and body type all play a role, but we can consciously relax these muscles (upper trapezius, levator scapulae, rhomboids) throughout the day to ease this constant tension. Constant tightness/tension from a slight muscle contraction held for long periods of time in this area can cause feelings of stiffness, tightness, fatigue, burning, restriction, cramping, sensations into the shoulders and arms, headaches.. any of which can increase irritability and overall discomfort.