Rotator Cuff Tendinitis: This is the most common cause of shoulder pain, often resulting from overuse or repetitive strain on the rotator cuff muscles and tendons. It typically presents with pain in the shoulder and upper arm, especially with movement.

Adhesive Capsulitis (Frozen Shoulder): Characterized by stiffness and pain in the shoulder joint, adhesive capsulitis can follow an injury or develop without a known cause. It leads to reduced mobility of the shoulder.
Bursitis: Inflammation of the bursae, the fluid-filled sacs that cushion bones, tendons, and muscles near joints, can cause pain and swelling in the shoulder.
Shoulder Impingement Syndrome: This occurs when the shoulder blade puts pressure on the underlying soft tissues when you lift your arm away from your body, causing pain and limited mobility.

Myocardial Infarction (Heart Attack): Although less common, shoulder pain can be a referred pain from a heart attack, especially if the pain is radiating to the left arm or is accompanied by other symptoms like chest pain, shortness of breath, or nausea.
Pulmonary Embolism: In rare cases, shoulder pain can be associated with a pulmonary embolism, particularly if it's accompanied by symptoms like sudden onset of shortness of breath, chest pain, or cough.
Thoracic Outlet Syndrome: Compression of the nerves or blood vessels that pass into the thoracic outlet (the space between your collarbone and first rib) can cause shoulder and arm pain, among other symptoms.

Osteonecrosis (Avascular Necrosis) of the Humeral Head: A condition where the blood supply to the bone tissue is interrupted, leading to bone death and potentially causing shoulder pain.
Tumors: Both benign and malignant tumors can cause shoulder pain, though they are much less common causes compared to other diagnoses listed here.
Infections: Such as septic arthritis or osteomyelitis, which are infections of the joint or bone, respectively, and can cause severe shoulder pain, especially if accompanied by fever, redness, and swelling.