What is the reason for hair thinning on one side of the temple only?

Much hair loss runs in families and gallops in others. Do a family tree of hair loss by looking at as many relative as you can now and also old pictures.

Is there any evidence of scalp pathology there? If not it may be due to alopecia aereata, an autoimmune disorder. A dermatologist can determine the cause of the hair thinning,order appropriate tests, discuss treatment options & predict whether hair thinning is temporary or permanent.