The ultimate factor in me deciding on a specialty was that I wanted to be happy. That's complicated because one's perception of the actual practice of medicine changes from pre-med to med student to resident and then beyond as you progress through training and practice. That is probably the reason why I had a last minute change of heart from a surgical specialty to internal medicine with plans to subspecialize.
Here's the factors I thought would make me happy:
-lifestyle: Have to have somewhat of a normal, routine schedule so I can live a normal life. Know thyself. I am not a grind it out for a week and blow off steam the next week type of person. That's long term burn out for sure. I also would get regularly frustrated if I had days that went super long occasionally or was getting called into a hospital at 2am on a Sunday. I like being around friends and family. I like a little bit of "me time." I knew I would be a husband and father and dog owner. Working regular hours and essentially having nights, weekends, and holidays off facilitates this the best for me.
-money: "Money ain't everything, but not having it is." Grew up in the lower middle class. Worried about finances a lot. I wanted to be in a field where I could thrive financially. No shame.
-fulfillment: The meaning of this changed a lot overtime. I came into medical school thinking this meant I needed to do something "big" and cutting edge. Pretty much interventional/surgical stuff with a fair amount of risk. By the time I was finished with residency, I realized that I just wanted to make a patient feel better, increase the quality of life, and leave the exam room with both myself and the patient happy with how things went. If not...I wanted to get out of the room with minimal collateral damage and not have to keep seeing that patient again and again, in futility. You can do this in allergy while still maintaining the above criteria. Most of our patients are normal to high functioning, not responsible for their disease, and we can help them genuinely feel better. I pretty much never have to manage or workup pain-related complaints. I didn't find this to be as common in other non-surgical fields.
-Passion for the field itself: There's a lot of cool fields in medicine. I really like A/I. I also like the applied physiology and pharmacology of critical care, the tangibility and acuity of cardiology, and the structure and function involved with orthopedics. I also really enjoy the pathophysiology behind rheumatology, endocrinology, and other fields. So on and so on. A lot of people ahead of me told me "At the end of the day...it's a job" and I see that quite clearly now.
I've certainly got my share of burnout, bias, and jaded perceptions. This is just one dude's opinion.