Tread carefully with this question. Whilst the truth may be that you only get out of bed every morning in order to pay your rent, this is not what your potential employer wants to hear.
This question gives you an opportunity to discuss what has attracted you to this line of work and what inspires you to persevere through the tough times. In a sales role, this could be the adrenaline rush of meeting daily targets, whilst in a customer-service role, this could be the personal satisfaction you gain through helping people.