Your answer can be a little bit personal if you are comfortable with that. This question is designed for the interviewer to get to know you a little bit better. What makes you happy to go to work every day?

You could answer something like this: "The thing I enjoy most about being a plumber is being able to solve serious problems for home owners. The relief that they feel once their issue has been fixed is great and it makes me feel good to know that I helped them out."

You could also answer in a more concise was such as, "Every day is an exciting challenge because there is always something new to learn. That is what I enjoy most about being a plumber."