My left side of cheek is depressed. Doctor said they will add fat but they don't know the reason why it is happening? (photo)

Both implants and fillers in the cheeks can create good results for patients. Both can build up the cheeks and create symmetry. You must balance the pros and cons of each procedure and do what you feel is the best decision for you.
Fillers:
Pros: No downtime Reversible Quick
Cons: Temporary More expensive in long run
Implants Pros: Permanent Great for patients with poor bone structure Less expensive overtime
Negative: Risks associated with surgery Possible infection Downtime
I hope this helps.

Some volume loss is natural with age. Volume of the face can be created with well-placed fillers (Voluma, Radiesse, Bellafill) and Sculptra. This would allow you to try on the look before committing to a permanent option like fat or implants. I recommend getting a formal evaluation with a facial sculpting expert. Best, Dr. Emer

 In our practice, we do not perform fat injections to the cheeks for a variety of reasons. When patients have flat maxilla, a flat cheek profile and loss of soft tissues in the cheek,  we perform placement of cheek implants. Cheek implants are manufactured in a large array of thicknesses, sizes, shapes. They can be custom carved when necessary. The implants themselves are usually placed through a small incision through the mouth under a  brief general anesthesia as an outpatient procedure. For many examples, please see the link and the video below

You need to be medically evaluated first.  Go to someone familiar with congenital or acquired hemifacial asymmetry.  In our experience from many similar patients over the years, there can be several causes.   A double board-certified Facial Plastic surgeon and ENT would be your best bet.  Best wishes.

I agree with you; i feel it would be best to diagnose the cause of the progressive depression of one side of you face before having cosmetic surgery. I recommend you consult with an oral surgeon or maxillofacial surgeon.