If I got paid to watch netflix and eat ice-cream all day that would be motivation enough. But sadly that job isn't what you are applying to so you need to think on your toes. Are you motivated to give 110% to satisfy the patient? Do you look forward to your weekly mentoring sessions from a fellow co-worker that is helping you identify your goals? Do you set personal goals to see how you can improve your efficiency each day? Maybe your learning a new software program and your desire to be programs go-to person in the office is your driving force? Whatever motivates you, do your best to relate it to the position you are interviewing for. Don't be afraid to be passionate about it- the interviewer will love it.