There are several reasons for pain inside the left rib cage: 1. It is due to trauma to the intercostal muscles or frequent exposure to cold, resulting in the formation of local inflammatory edema as well as bruising and swelling, which stimulates the peripheral nerves around it and causes pain. In addition, the intercostal nerve can be stimulated by the edema of the intercostal muscle, leading to more pronounced pain. 2. Pleurisy, such as inflammation of the pleura due to bacteria, viruses, Mycobacterium tuberculosis and other inflammatory edema, can also stimulate the peripheral nerve, leading to pain and often combined with fever, dyspnea, cough and other symptoms. 3. Heart problems, such as angina pectoris and myocardial infarction. The pain can be caused by angina pectoris and myocardial infarction. 4. problems with the lungs, such as lung tumors, which will gradually increase in size and produce extrusion and stimulation of the surrounding tissues, which will also cause pain in this location.