Neck pain on the left side may be caused by pillow fall, cervical spondylosis, cervical lymphadenitis and so on. 1. Falling pillow: generally related to pillow and sleeping posture, if the pillow is too high, too low or too hard when sleeping, it will make the neck in the state of hyperextension or hyperflexion, so that the muscle strain and spasm; if sleeping in an improper posture, it may also cause excessive head muscle deflection, resulting in the cervical vertebrae skewed to the right, which can be manifested as the left side of the neck pain. 2. Cervical spondylosis: degenerative changes of cervical intervertebral discs and secondary degeneration of intervertebral joints. Cervical disc herniation, cervical spondylolisthesis, etc. can lead to the left side of the spinal cord, vertebral artery, nerves and other pressure, resulting in the left side of the neck pain, dizziness, headache and other symptoms. 3. Cervical lymphadenitis: It is mainly caused by infection, and the causative bacteria are mainly Staphylococcus aureus and hemolytic streptococcus. Infections in different parts of the body invade the corresponding areas along the lymphatic vessels and cause inflammation. The main manifestations are swelling and pain in the lymph nodes of the neck, and may also form abscesses, superficial skin congestion and swelling. There may be other causes of left neck pain, and you should consult a doctor to clarify the exact cause and treat accordingly if necessary.