This is a more personal question that is best answered using a specific life event.
"When I was in junior high, our class had two special needs students. I would see their aids assisting them throughout their day and it was really beautiful to see how important their role was in that student's life. I had always wanted to be a teacher but this opened my eyes to the opportunity of specializing in Special Education."
"When I volunteered as a job coach helping developmentally disabled individuals, I got a lot of satisfaction teaching my clients how to do a job that they enjoyed. I realized that I wanted to do more than that and to help more individuals at once."