This question is practically begging you to highlight your positive attributes. So don't give a vague, generic response - it tells them very little about you. Instead, try and use this question as an opportunity to give the interviewer some insight into your character, and use examples where possible.

"I've always been motivated by the challenge of a tough situation. When I volunteered as an activities aide, I came across a particular patient who refused to participate in any activities. I was told he was 'always gruff and impossible.' I talked with him and was able to find something we had in common, which calmed him down. After the patient saw me make an effort to better understand his situation, he became much more agreeable to my aide. I love facing and overcoming challenges on such a personal level."