The shoulder lets you perform a myriad of sports skills, such as throwing a football and serving in tennis, because it has a greater range of motion than any other joint in the body. However, to achieve this range of motion, the shoulder sacrifices some stability.
In the illustration below, notice how the entire arm is attached to the scapula (shoulder blade), which in turn is attached to the clavicle (collar bone). This provides little structural support to the joint, so it must rely on upper body muscles for stability.
To build this stability, most athletes focus on training their deltoids and rotator cuff muscles. These are important, but you also need to develop the muscles in your upper back that support the scapula, like the trapezius and rhomboids.

In fact, the muscles that move the scapula are the source of many shoulder movements. Weakness in them can lead to problems in other areas of the shoulder joint. According to a study in the Journal of Orthopedic and Sports Physical Therapy, weak scapula muscles cause three critical problems:

• Increased stress to the front of the shoulder
• Increased risk of rotator cuff injuries
• Decreased performance

To strengthen these muscles, regularly perform exercises that target the upper back. When doing so, remember to retract your shoulder blades and relax your upper traps by keeping your shoulders as far away from your ears as possible. Pull with your back muscles instead of your arms, and imagine that you are squeezing an orange between your shoulder blades throughout each movement.
DB Single-Arm Rows
TRX Inverted Row
Half-Kneeling Face Pulls
(Videos Above)
Photo:  Chris McGrath/Getty Images
Sources:  Paine, M., & Voight, M. [1993]. The Role of the Scapula. Journal of Othopedic and Sports Physical Therapy, 386-391.; livestrong.com