Rotator Cuff Tendinitis: This is the most common cause of shoulder pain, often resulting from overuse or strain of the muscles and tendons surrounding the shoulder joint. Activities that involve repetitive overhead motions or heavy lifting can lead to inflammation and pain.

Adhesive Capsulitis (Frozen Shoulder): Characterized by stiffness and pain in the shoulder joint, adhesive capsulitis can significantly limit the range of motion. It often occurs after a period of immobility or less commonly, without a known cause.
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.
Labral Tears: Tears in the labrum, the cartilage that surrounds the socket of the shoulder joint, can cause pain and instability in the shoulder, often due to injury or repetitive motion.
Osteoarthritis: Wear and tear on the cartilage of the shoulder joint can lead to pain, stiffness, and limited mobility.

Myocardial Infarction (Heart Attack): Although less common, left shoulder pain can be a referred pain from a heart attack, especially if the pain is accompanied by other symptoms like chest pain, shortness of breath, or nausea.
Pulmonary Embolism: Sudden onset of shoulder pain, especially if accompanied by difficulty breathing, chest pain, or cough, could indicate a pulmonary embolism, which is a blockage of an artery in the lungs.
Gallbladder Disease: Referred pain from gallbladder inflammation or stones can sometimes be felt in the right shoulder but less commonly in the left, though it's still a possibility, especially if accompanied by abdominal pain or jaundice.

Pancoast Tumor: A rare type of lung cancer located at the top of the lung, which can cause shoulder pain due to local invasion of the tumor into the brachial plexus or ribs.
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 pain in your shoulders and neck and numbness in your fingers.
Polymyalgia Rheumatica: An inflammatory disorder that causes muscle pain and stiffness, typically in the shoulders and hips, and is often associated with giant cell arteritis.
Referred Pain from the Spine: Certain conditions affecting the cervical spine, such as a herniated disk or spinal stenosis, can cause referred pain in the shoulder area.