Headaches on the left side of the head are commonly caused by a direct blow to the head or other minor injury, but can also be caused by ear infections, tooth infections, or migraines. This type of headache may be accompanied by other symptoms such as localized swelling, nausea, and fever. Headaches on the left side of the head aren't necessarily cause for concern, but left-sided headaches with symptoms such as weakness on one side of the body, blurred vision, and seizures can be signs of symptoms such as: More serious conditions, such as brain tumors and severe brain damage. In these cases, imaging tests such as CT or MRI may be ordered.
If headaches on the left side occur frequently, a doctor should be consulted for further evaluation, to rule out serious disorders and, if necessary, to initiate treatment. 

Reason Behind left side head pain:

The causes of headaches on the left side range from lifestyle habits such as skipping meals to overuse of medications. 

1. Factors behind our Lifestyle:

If you suffer from frequent headaches, lifestyle changes can greatly reduce your symptoms. Any of these factors can cause headaches. 

 1. alcohol

 2. skipping meals

 3. stress

 4. lack of sleep

2.   Allergies:

    Headaches are often a symptom of respiratory infections such as colds and flu. Fever and sinus congestion can cause headaches. Allergies cause headaches due to sinus congestion, resulting in pain and pressure on the forehead and behind the cheekbones. 

3. Over use of medicine:

Taking medications used to treat headaches more than 2-3 days a week can make headaches worse. It happens almost every day and the pain starts when you wake up in the morning.
Drugs that can cause overuse headaches include:

     aspirin
      Paracetamol (Tylenol)
      Ibuprofen (Advil)
      Naproxen (Naprosyn) 

4. Neurological causes:

Occipital neuralgia:
            The occipital nerve runs from the top of the spinal cord through the neck to the base of the skull. Irritation of these nerves may cause severe, sharp, stabbing pain in the back of the head or base of the skull. The pain lasts from seconds to minutes.
Giant cell arteritis:
                 Also called temporal arteritis, this condition is caused by inflammation of blood vessels, such as the temporal arteries on the sides of the head. Symptoms include headaches, jaw, shoulder and back pain, and blurred vision.

Trigeminal neuralgia:
                 This condition affects the trigeminal nerve, which gives sensation to the face. A sharp pain like a sudden impact occurs in the face.