Why do you want to be a volunteer in our organization?
"Your mission is to deliver resources and make a difference in the lives of those underserved. I cannot think of a better cause to be a part of, and I'm excited about the opportunity to be on your team!"
"I am passionate about being in the position to help others, especially when I can utilize my skills to make a difference in the life of others."
Share what you know about the organization and the aspects of the role that you're most excited about. Enthusiasm, based on knowledge, will go a long way. This is a chance to 'wow' the interviewer.
Discuss what your underlying motivation is. If you need volunteer hours to get into a college or graduate program, tell them about the program and share how it relates to their organization. They aren't just looking for passion; they want to know you are invested in their cause!