What can be the cause of constant ringing in my ears?

Usually hearing loss. The most common cause of tinnitus is hearing loss. Tinnitus can also be caused by ear injuries, medications, stress, jaw spasm and many other reasons.
Tinnitus. Chronic tinnitus has many causes, but commonly is associated with damage to the inner ear and/or associated hearing loss, and the brain's method of compensating for this inner ear damage / hearing loss. You should have your hearing tested by an audiologist and see an ENT to make sure nothing else is going on.