Your reasons should relate both to the occupation and the particular company interviewing you. Your interests in any of the following could prove useful to mention: working to tight deadlines, working under pressure, meeting targets, getting the job done, selling, making deals, being competitive, presenting to others etc.
Don't feel you have to limit your answer to just the occupation alone. Give reasons why you find the particular firm attractive. It is often a good idea to broaden the scope of a question and turn it into more of a normal conversation.
Include any examples of selling from your university days. Did you effectively advertise the films at the film society, for example, or increase the membership, or attract many more volunteers, or sell lots of tickets for an event? etc.