I was preparing for a Starting Strength Press Camp last month, and in the process of reviewing anatomy, was reminded of how unstable the shoulder joint is. In order to provide us bipedal humans with useful upper extremities, the glenohumeral joint allows lots of movement. In order to allow lots of movement, it is a ball-and-socket joint, but the socket is so shallow it might as well be called a ball-and-saucer joint.
Imagine a wooden dowel articulating with a teacup saucer. Not very sturdy, is it? It's very mobile, but also very easy to dislocate. How is this prevented? First, through a sheath of connective tissue connecting the humerus to the scapula:
Then, through the muscles of the "rotator cuff", which do not really do much to rotate the shoulder, since their moment arm on the humerus is too short, but mostly serve to provide joint stability. You can see them quite well in this illustration:
See the supraspinatus, infraspinatus, teres minor? You can't see the subscapularis, since it's under the scapula. Note how they attach right on the head of the humerus. They can't really move the humerus much, but they can keep the humerus firmly seated in the socket of the glenoid fossa (the socket of the ball-and-socket joint).
My epiphany happened as I gazed at figure 4-20 in Starting Strength:Basic Barbell Training. It's on page 116. Go ahead and get out your copy. (You don't have a copy?) When you deadlift, the barbell tends to want to pull your humerus down in the direction of gravity, out of the glenoid fossa. What prevents this? The muscles of the rotator cuff, as well as the ligaments and tendons of the joint, which contract isometrically to stabilize the joint that the barbell is trying to pull apart.
The second part of my epiphany is considering the nature of the force in a joint under tension. In such a situation, 100% of the force is transmitted through the connective tissue. The bones are just links in a chain, and every link in a chain bears the complete force of the load. Consider a series of blocks connected by cables. If you stack the blocks one on top the other, the blocks carry the load, and the cables are unused. If, however, you hang the blocks from a hook, the blocks and the cables both carry the entire load. This means that when you deadlift 500lbs, the entire load is transmitted through the ligaments and tendons, and thereby to the muscles of the rotator cuff, as well as the biceps and triceps, which also traverse the shoulder joint. Your bones are not hooked together. They are chained together, and under tension the chain gets all of the load.
The third part of the epiphany is an article by Hartmann: "Analysis of the Load on the Knee Joint and Vertebral Column with Changes in Squatting Depth and Weight Load." What does an article on squats have to do with the shoulder? He points out various studies which have shown that the patellar tendon in olympic weightlifters is larger and stronger. The ACL of professional weightlifters also are bigger and stronger. The connective tissues adapt, just like muscles. What is true of the knee joint should be true also of the shoulder joint.
In other words, if you want a strong and stable shoulder joint, you need to lift heavy stuff. Of course, you need to press overhead, but I argue that you also need to deadlift. The deadlift provides tensile stress pulling the joint apart, thus provoking adaptations that keep the joint together. If you deadlift, your rotator cuff muscles will get stronger doing their proper job of isometrically contracting to protect the shoulder, and the ligaments and tendons of the joint will adapt to the load by getting stronger.
If I'm right, you need to deadlift. How can you learn to do it? You can buy The Book (SS:BBT), but you can also look at this video on the basic setup for the deadlift. Follow the steps exactly, and you'll be on your way.