The interviewer wants to know how they can motivate you during the tougher days. Do you need words of appreciation? Public accolades? A pat on the back? What keeps you going when things get rough? This would be a great time to share how serving others is a motivation in itself. The interviewer is looking for qualities they want to see in a member of their team. Service, interaction with the people you're serving, having fun with your team... those are all excellent examples.
"I'm not looking for a specific reward from your organization, simply being involved in a team setting, making friends, and knowing that I am making an impact on those in need, will be rewarding enough!"
"I am internally motivated. When I feel like my time and energy is making a positive difference, it makes me want to keep going."
"Truly, I don't thrive on public recognition. I do appreciate a little pat on the back, or kind words in private, if you do see me performing well."