Almost every interviewer will ask this question and, “Because I am the best candidate for the job,” is not the right answer. The interviewer wants to know why you are the best candidate. You will want to talk about your skills and accomplishments as well as what you can can do for the company or firm. You might say, for example, “Because I have the experience you are looking for and my creativity and problem solving abilities can make this company money” or “Because my research and organizational skills, combined with my ability to get things done would make me a valuable asset to this firm.”