You had to have an emotional response.

It was difficult work. Bosch never got accustomed to viewing crime scenes. He had been to hundreds of them and seen the result of human inhumanity too many times to count. He always thought that if he got used to it, then he had lost something inside that was needed to do the job right. You had to have an emotional response. It was that response that lit the match that started the fire of relentlessness. (Connelly, Michael. The Crossing, kindle location745-747)