A psychotherapist's work is catalytic, fostering the patient's unconscious mind in its quest for greater emotional health and improved functioning. While this cannot be guaranteed, it usually happens and patients leave therapy satisfied. Likely without achieving as great joy as they wished but that is not a reasonable goal since life gets in the way.
But there is also sadness for the doctor who, with rare exception, never learns how their patient's life progresses, like reading a novel with missing chapters. Unlike parents who, after surrendering their grown-up child, do learn of their offspring's continuing life.