Although an interviewer will rarely ask it this directly, one of the key questions they need to get answered during an interview is why they should hire you. They are likely interviewing multiple people for the position. To get them comfortable that you are the right person for the job, you’ll need to demonstrate that:

   1. You are genuinely interested in the job, company and/or industry as demonstrated by past or current experiences
   2. You understand the job; and
   3. You have the skills needed to be successful

Interviewer: Why are you interested in this job and what makes you a compelling candidate?
 
WEAK ANSWER
I have a few older friends who have gone into pharmaceutical sales and they really like it, especially the flexibility the job offers. I don’t really see myself sitting behind a desk all day, so this job really appeals to me. I’m also interested in being able to develop strong sales skills and getting rewarded for my success.
 
STRONG ANSWER
Over the past two years I’ve come to realize that I have some pretty good sales skills. This semester I’ve worked part-time for a local real estate developer going door-to-door to local businesses to promote a new hotel in the city. I have far exceeded the goals and found that I really enjoy developing a strategy for how to get to the decision maker at each business, and how to tailor my pitch.  In researching sales opportunities, I feel that my demonstrated ability to sell, and my organization and communications skills are a great fit pharmaceutical sales  

Analysis
The Strong answer clearly demonstrates an understanding of the skills that are important for the job and compelling reasons why the individual would be interested in the job and good at it.  
 
In the Weak answer, the interviewee’s interest in the job seems centered on shallow reasons (friends in the job, flexibility, not a desk job). He also doesn’t communicate a deep enough understanding of the job and why he is a good candidate. As a result, the candidate leaves the impression that he/she doesn’t really understand the job or have thoughtful reasons for why they should be hired over anyone else.