What is the best way to answer this job interview question

This is difficult to answer. Are you applying for a job now in your field? if so, i would suggest that you say, "I wanted to find the optimal job in my field so I can apply my knowledge, experience and skills i have learned to benefit my employers needs." If this is a job outside of your field, "I decided early in my career that my previous work experience was not the correct path for me. I am now taking those skills and applying them in XXXX field." Provide more details regarding this job. Most interviews are about how you say things.