The first answer that comes to mind is

The first answer that comes to mind is, "Because you have an opening, and I need a job." While this may be the truth, it is not an answer that will get you points in a job interview. The interviewer is looking for an answer that indicates you have thought about where you want to work, and are not just sending out your resume to any company that has an opening. Doing research on the company and industry before the interview will make you stand out as a more informed and competent applicant.

It's all in how you say it

Annette is the first candidate interviewed for the position of HR Manager. When she is asked this question she replies,

"I always wanted to work for this company. I not only love the product but, have used it for many years. This job would be perfect for me, a real opportunity for me to grow and develop."

Annette's answer begins well, but then shifts to what she can get out of the experience instead of what she has to offer. The answer would be stronger is she were to indicate that she had done research on the industry and the company, and could discuss more than her experience with the product.

James is another candidate and answers more directly.

" Based on the research I have done, this company is an industry leader. When I visited the website, I found some very impressive information about future work planned here. I was also impressed with the backgrounds of the founders, and the current financial statements. This is a company I have been looking for, a place where my background, experience, and skills can be put to use and make things happen."