Why law?

What should non-law students say to persuade an interviewer that they are serious about a career in law?

Even though I studied Economics at university I’ve decided to pursue a career as a City lawyer. I’m pretty sure in my own mind that this is the right road for me but when I was asked the question “why law” during a recent vacation scheme interview I completely messed up my answer. What should I be saying to potential interviwers to persuade them that I’m serious about law? All suggestions welcome. Thanks.