Every individual you ask will have a different answer to this question. There is really no right or wrong answer. However, being tactful is always best, which means do not answer with 'My favorite thing about what I do is my paycheck.' Find something about your job that is interesting, something that challenges you or makes you feel accomplished. That is what the interviewer is looking for from you.
"I like the challenge of diagnostics. I like the satisfaction of saving an animal's life. I like the things that we do to make animals live longer, live more productive lives, spaying and neutering, and all healthcare."
"I love the feeling of having someone who is able to thank me for caring for their animal. To real animal lovers, their pets are part of the family. Being able to give that pet back to them for a longer time is a great feeling."