Why is my rib on the left side sometimes sore?

Many causes: Most concerning left sided pain is associated with the heart, but many other causes include trauma, costochondritis, anxiety, stress, gastroesophageal reflux, and sometimes from abdominal organs such as bowel. Stomach , gall bladder, etc. In the chest pleuritis, pneumonia, pulmonary embolism, collapsed lung etc. You should consult physician for physical examination and testing if appropriate.