Answer:
Hello! This a very individual question. Every applicant will have a different answer to it.
To write your own answer, look back at your school years and think of teachers and mentors who thought you and influenced you along the way. Maybe, some of them helped you to discover and develop your interest in the field you have chosen? How did it happen? Did they suggest you to read a certain book, work on an extracurricular project or do and additional research? Recall a specific situation and work your answer from there.