I've found that quite a few people who I met who wanted to do medicine at the start of this year (AS level) have since changed their mind to other careers. I think when you were younger and university seemed like a long way off, medicine didn't really seem like that much of the commitment. As people realised what the career actually entailed, how competitive it was etc, they changed their mind.
It's quite weird because I didn't decide I wanted to do medicine until quite late, but now that I've realised it's what I wanted to do, I honestly can't imagine applying for everything else. I will openly admit that the (relative) job stability and comfortable salary are part of the overall reason for wanting to do medicine, but I think the main reason I want to be a doctor is because I literally cannot think of anything else that I would rather do, anything else that I believe that I'm truly suited for.