The most common question is quite obvious; "why do you want to be a doctor?"
Now, the answer to this might be simple in your mind but you have to remember that your interviewers will want to play devil's advocate. So simply saying, "I enjoy the science of the human body" is not sufficient because they will turn around and say "well why don't you go into research?". The same is true if you say "I enjoy caring for people". The interviewer will usually then say, "Well, why don't you train as a nurse?".
The trick is put all your reasons together as a combination. It's also worth throwing in that you want to challenge yourself to the best of your ability and becoming a doctor is the perfect fit. Remember, whatever your reasons, be sure that you can back it up with evidence!
This is by far the simplest question they will ask but also the most difficult. Be prepared to justify all of your reasons.
Finally, don't be worried that they are testing you; becoming a doctor is one of the most grueling courses you can do- they need to be sure that you really want it!