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. The pain is typically localized to the shoulder and may radiate down the arm.

Adhesive Capsulitis (Frozen Shoulder): Characterized by stiffness and limited range of motion in the shoulder, often following a period of immobility or trauma. Pain is usually aching and constant.
Subacromial Bursitis: Inflammation of the bursa that cushions the rotator cuff tendons, leading to pain and swelling in the shoulder. It can be caused by direct blows to the shoulder, repetitive movements, or conditions like rheumatoid arthritis.
Shoulder Impingement Syndrome: Occurs when the tendons of the rotator cuff are compressed as they pass under the acromion, leading to pain and limited mobility. It's common in individuals who frequently lift their arms above their heads.

Myocardial Infarction (Heart Attack): Although less common, shoulder pain can be a referred pain from a myocardial infarction, especially if the pain is radiating to the arm, neck, or jaw, and is accompanied by other symptoms like shortness of breath or chest discomfort.
Pulmonary Embolism: Can cause referred pain to the shoulder if the embolism irritates the diaphragm or the pleura. Other symptoms might include sudden onset of shortness of breath, chest pain, or cough.
Gallbladder Disease: Referred pain from gallbladder inflammation or gallstones can sometimes be felt in the right shoulder, often accompanied by abdominal pain, nausea, and vomiting.

Pancoast Tumor: A type of lung cancer located at the top of the lung that can cause shoulder pain by invading the brachial plexus or the shoulder structures. It's often accompanied by other symptoms like arm weakness, Horner's syndrome, or weight loss.
Osteonecrosis of the Humeral Head: A condition where the blood supply to the humeral head is interrupted, leading to bone death and potentially causing shoulder pain. It can be a result of trauma, long-term use of corticosteroids, or excessive alcohol consumption.
Infectious or Inflammatory Arthritis: Conditions like septic arthritis or rheumatoid arthritis can cause shoulder pain, though they are less common causes. They are often accompanied by systemic symptoms like fever, malaise, or swelling and redness of the affected joint.