Shoulder joint is a connection between your torso and arm and thus is provided by a vast range of movement, which makes it more prone to injuries. Any disturbance in the intricate balance between the mobility and stability ratio of the shoulder can lead to pain.
Injuries to the surrounding muscles, ligaments, tendons, bursa and joints due to gym exercises, sports injuries, activities related to lifting weights etc can lead to shoulder pain. The pain area varies depending on the cause, pain can be constant or activity based.