You typically travel alone so family, friends and the rest of your world stays behind. That is probably the biggest negative, the time requirement that the job takes and the fact that you're on the road a lot. Nothing that I did as kid prepared me to be a veterinarian. I thought that I was going to go to medical school. Somewhere in there I decided that I really didn't want to be a physician, and I thought that a career in veterinary medicine had societal benefits that are equal to, or may exceed, what physicians contribute.
Animals are one of the few things that truly make people happy.
They are the experimental subjects, and taking care of them, keeping them healthy so that the results are meaningful, saves peoples lives. I think the most rewarding thing in a career in laboratory animal medicine is knowing that what you are doing may keep a small child alive, or may allow them to find a cure for cancer, or a better treatment for heart disease, or develop preventative measures which keep people healthier and out of the hospital. To prepare for a job like this you have to go to veterinary school. It is really hard to say how to prepare because again, I didn't go looking for this job, it came looking for me. I had no way to prepare for working for the space agency except to have a good basis in veterinary medicine, a good basis in animal husbandry and laboratory animal science.