Here's a quick anatomy lesson: The shoulder complex includes the humerus, clavicle, thoracic region of your spine, rib cage, and perhaps most importantly, your scapula (or shoulder blade). The upside to all this complexity is a huge range of motion in our upper body (think: nailing wheel pose and throwing curve balls).
The downside: The whole area depends on ligaments and muscles for stability—as opposed to the "ball-and-socket" stability our hips get. When those muscles get overused (or misused), we're left with limited motion and stiffness. All four [shoulder] joints need to be strong and working in concert in order to enjoy a healthy upper body.
The foundation for shoulder health includes proper (and regular) stretching exercises.
The following article rounds up 16 of the better shoulder stretching exercises to consider adding to your daily routine: