Recently, I was talking to a 3rd year student, and he was trying to figure out what to do with his life. He was still a little lost, like most people in their 3rd year. Everything either seems fun or horrible. Everyone expects that moment of, 'This is what I want to do with the rest of my life!' but in reality, few of us get that moment. I can't tell you when I decided on internal medicine. I just did.
I tell most people that they should go with the pathology. Whatever diseases interest you are probably things that will continue to interest you, because the patients can't all be winners. This student told me that he wanted to do ER because he didn't want to see the same thing over and over. I was polite enough not to laugh in his face.
The truth is that every specialty has its bread and butter. For me, that's hypertension and diabetes. For the surgeon, it's gallbladders and appendices. And all specialties have their chronic patients. You can't choose a specialty aiming to avoid these chronic folks or to dodge the routine diseases. The only way to dodge that stuff is to go into a field without direct care: radiology, anesthesia, path...
When I was trying to decide, I posed a simple but profound question to myself: what is my staple diet? Sometimes I get a taste for Mexican, but I can eat rice every day for the rest of my life. And that's what choosing a specialty is like to me. What can I see again and again and always find satisfying? That's the specialty for me.