This applies to medicine as a whole, but in particular to oncology. A lot of people "question my sanity" when I tell them that I want to be in a field like oncology. "Wouldn't it be depressing to be treating cancer patients all of the time, especially when ones that you become close with ultimately have unsuccessful treatments?"
It is all about how one looks at things. For starters, maybe cancer as a disease is a depressing thing but oncology as a specialty is amazing. Before 1950 or so, all cancers had the same prognosis: zero, give or take a few hundredths of a percent. Now, we save people all of the time. Colon cancer can be resected and cured with surgery and chemo. Testicular cancer has a 95% survival rate when only a few decades ago it was 5%. Many pediatric leukemias we can treat with some efficacy. For cancers we can't cure, we can certainly make the remaining life better. In oncology, is it isn't about who you lose - its about who you don't. Those are the cases where, after, you can look Mother Nature in the eyes and say "we beat you."
The other thing about oncology, although this applies to other fields as well (for example, transplant surgery, which I am on right now), is the effect it has on me as a person. Its so easy to go home at night and be tired and think, "I want to do nothing", or "I am too tired to go on a run." Then you remember the stage IV cancer patient who is bed-ridden and beyond cure, without long to live. Or you might remember the young guy who is stuck in the ICU. He already had a liver transplant once to save his life, but his immune system has systematically destroyed it despite our best efforts. Each day we see the new numbers, each day more abnormal, as the liver functions less and less.
Remember those things and all of the sudden, you have the energy to go for a run or to go out and socialize - because you can. Because you are so aware that someday, you won't be able to. Think forward - many of us will be in that very position some day. What would we give for the chance to come back and be 28 and be healthy enough to do those things? If that time comes for me, I will smile when it does, because I will know I did. With the right perspective, oncology is an incredibly uplifting discipline.