Answer:
Any of the healthcare professions, physician, nurse, or any other, are difficult enough just when taking into account schooling. When a person decides to go into these medical fields, whichever it may be, without enthusiasm, the amount of studying and preparation alone, will overwhelm them.
But once in the field, once working, a medical professional, of any field, faces nothing more and nothing less, than having the safety and integrity of human beings in their lives, from conception, to their natural death. That fact can become even more overwhelming for a professional, and without enthusiasm, this effect will come much faster, and have dire consequences.
So, enthusiasm helps a medical professional advance on the job, by providing the necessary motivation to face any and all obstacles, regardless of which they may be, and focus on improving skills, because the professional wishes, thanks to motivation, to grow professionally, and be better professionally. That´s how enthusiasm will help a medical professional advance on the job.