There are a lot of causes for shoulder pain. The first thing that you want to understand is that the shoulder is a very complex joint. It is what's called a ball and socket joint meaning there is a socket which is part of your scapula. Then you have the long arm of your arm called the humerus. And that's the ball.
So, inside underneath here you have a mechanism of a ball and socket which means it does allow for a fine number of degrees of movement. For instance, it can horizontally ab and abduct, it can flex and extend, it can rotate, and it can also do them in combination. The point of all that is that it relies heavily on the muscles and the ligaments and the soft tissue for stability. It's an unstable joint, so over time using your arm incorrectly with reaching, with lifting, and with carrying can lead to some strains in the muscles.
Another cause of shoulder pain is the idea that, because of our sitting posture and our posture in general, we tend to round our shoulders forward too much. As a result the structures in the front side of our shoulder tend to get shortened and stronger, and the structures in the back side of the shoulder end up getting weakened and lengthened too long during the day. And as a result there's a poor balance. Every time we go to reach, or carry, or reach for something we're going to create an imbalance in those muscles working. We don't know it at the time, but eventually that can lead to some muscle strains or, worse, some joint pathology like degenerative changes in the joint.
The other problem with the shoulder is that it does rely heavily on having the person use the arm to the fullest meaning using all the range of motion that it's allowed to do. Unfortunately, our daily life doesn't require that. We tend to use it in front of us. We tend to use it down here. But we're rarely up here doing activities. As a result we lose some range of motion. We lose the ability to do it. It becomes very restricted. That can then also lead to further degeneration inside the joint, further breakdown, and the need for care.