It's important to remember that an interviewer is not expecting you to say that there is nothing about your job that you dislike. In fact, doing so would most likely lead him to wonder if you are being honest about your other answers. It's ok to have something that is not your favorite thing to do, just make sure it is not a core skill/requirement that is listed in the job description.
"I can easily answer that question. My least favorite thing about being a veterinarian is the fact that, unlike a medical doctor, my patients can't verbally communicate with me. So, if an animal is very sick and no one saw an accident or injury, I am often left using symptoms and the process of elimination to determine the diagnosis in order to prepare a treatment plan."
"My least favorite thing about being a veterinarian is that there doesn't seem to be enough hours in the day for me to do everything I want to do. I am one of the ones who are likely to stay late just to watch and make sure a sick dog doesn't take a turn for the worse, or to give 'Fido' an extra walk before heading home."
"I suppose my answer to this changes from time to time. It's not so much things that I dislike, but things that I wish I could make better. For instance, I wish every little grandma who brings her 15-year-old cat in could take it home with her instead of hearing that the cat is not going to make it. I wish I had x-ray vision and a magic wand and could fix every broken bone that is the result of a pet being struck by a car because it didn't have a secure yard to play in. Those may seem like minute things to say, but in the world of veterinary medicine, I am taking care of someone's best friend every day."