Your reasons could take various approaches: why you want to join the legal profession; why you want to become a solicitor as opposed to a barrister; why you are interested in the particular firm interviewing you. It is a good idea to give an account of any practical experience, such as work placements, that has given you an insight into law in practice. You can also take the opportunity to put over your personal qualities and interests that relate to the work, such as communication and interpersonal skills, initiative, numeracy and flexibility.

Try not to simplify your answer. In other words do not suggest that there is one reason, and one reason alone, for your choice. You should be firing on all fronts in answer to this question. The interviewer needs to be convinced that Law is something you really want to do and have thought about seriously.