WHEN Dr. Robert Greig, a thirty-eight-year-old war veteran who graduated in dentistry last spring, faced his first patient the determination which had carried him through six years of hardship as a student suddenly deserted him. Excusing himself he left the patient, a woman, and walked from the sun porch, where he had set up his office, through the living room where his four children were playing to the kitchen where his wife, Velma, was baking.