Question

what is the reason behind the cause of headache

Answer 1

Headaches are caused due to excessive using of Brain and sense organs like eyes.
The different cells (Sensory and Motor ) if used extensively and continuously and without rest, it exhausts them and brain feels tired so it Aches and that is called as headaches.  ):=

Answer 2

Most headaches happen in the nerves, blood vessels, and muscles that in a person's head and neck. Sometimes they swell, tighten, or go through other changes that stimulate the surrounding nerves or put pressure on them. These nerves send pain messages to the brain, and this causes headache.