Well, we all know the definitive answer to this question especially ensconced in an interview room! You have probably heard the terms "live to work" and "work to live"? Well you would be very fortunate to be in the echelons of the former but for the most of us it is the latter. Whatever is your choice for this answer, try to avoid saying money for this job. A job that requires handling a lot of money everyday needs someone who is there for the love of that job and not for the money. Of course there will be a lot of tips as well but try to focus on the good parts of job description and mention those in your answer as your motivation.