It is not the sickness and the injuries that bother me, although there are sometimes that I get mad at the preventable injuries (like gunshots, and ATV accidents, and lawnmowers) - but like the previous poster said, the majority of the children I take care of get better and go home.
Kids are amazingly resilient, and they want to get better. I take care kids of kids after surgery and it's amazing how fast they bounce back after a surgery that would have put you or I out of commission for weeks.
We do a lot of ortho surgery (lots of spinal fusions, scoli repairs, etc) and I can't imagine something more painful than having a rod implanted into your back - but these kids ROCK IT OUT.
The sadness, for me, comes from the neglected children. The ones that are left to fight their battles alone. The ones that end up with me because their parents were too stupid to take care of them properly. The abused children. The kids that come in literally starving because their parents just don't FEED them. These are the cases that make me cry on my way home from work.
There will always be a child that you carry with you. I think that everyone has one where I work.... it is just the nature of the job. But it helps to know that these special children get love when they are under your care.