Almost any answer can be acceptable here, as long as it's true and you can connect it directly to your choice to pursue this field. You may have a personal experience that sparked your interest in social services, and it's perfectly fine to share that, as long as you explain how that personal interest led to your professional commitment.

"I have always enjoyed helping others, especially helping them discover resources they might not have found otherwise. When I was a student, I was constantly helping fellow students find the right tutor for chemistry or the right administrator to contact with their concerns. As I learned more about inequalities in our society, I began to feel a moral obligation to help offset those inequalities in whatever small way I could, so I became a social worker. I feel fulfilled when I can see a positive outcome for a client working with me or my team."