Setbacks often happen in environments like animal shelters, where you cannot always control or predict the events of a day. Everyone handles the stress and disappointment of setbacks differently. Discuss with the interviewer how you typically cope with setbacks in the workplace.
"Experiencing a setback is always disappointing, and can be a bit disheartening, but I understand that it happens from time to time. If I experience a major setback I will take a few moments to debrief with my manager and discuss what I could have done differently. Then, I move on!"
"Setbacks happen for a reason, and they do not affect me emotionally in the least. I fully understand that, when working with animals, one cannot always predict how a day will turn out. I am a very pragmatic thinker and stay focused despite the challenges that come my way."
"Earlier on in my career, I would take setbacks too personally and get frustrated when goals were not met. Now, I will go through a mental rundown of what I did wrong or what I could have done differently. It can certainly be emotionally taxing, but I also learn a ton from it."