wow, a very broad and complex question which I'm sure will come up a lot in the future for me anyway, as it often comes up in my own mind.
I guess at first it was for shallow reasons. It was because I loved science- I remember my dad bought me various fact books as a child and I kept going back to the one about the human body and reading all the facts about how different colours of hair had different amounts of hair, fight or flight, the best age for females and males to run marathons etc. I loved the diagram of skin and the eye. I absolutely loved learning about it all throughout school and found it all very exciting and stimulating.
I found out that doing medicine was learning about this and applying it in a mentally stimulating way so, at first, that's why I wanted to do it. It doesn't hurt that it's very well respected, either, and as someone who's been doing pretty well at school people always expected me to get a good career or go to a good university etc (Never for money, that being said.. I am happy with just enough money and have never been very 'well off' in any sense, but I've never longed for more money in a career and it baffles me when people say they want to do medicine for the money).
Then I realised it was a lot more than just liking science. I talked to so many people and read so much about it (quite recently, actually- within the past year) and found out that, more than anything, being a doctor meant having empathy, care and compassion. I questioned myself a lot, and still do, about if I am good enough at caring and being empathetic. I thought whether I could communicate to people well enough.
I've not done much volunteer work so far, only a little in a respite care home for severely disabled people, but I've loved it and want to do more. I've overcome lots of issues I've had with communication in the past and have learnt to love speaking and listening to different people. I love how the nhs and healthcare is such a brilliant community.
So basically medicine is something that I have considered for a very long time. My interest in it has developed over a number of years and I hope it will continue to develop. But it combines everything that I love- science, problem solving, ethics, mental challenges, communication with others, healthcare in general. I've got to the point where I love the idea of it so much I'm ridiculously connected to it and couldn't really imagine myself doing anything else. I'll do more work experience/ volunteer work in the next year and see if I still want to apply but yeah, thats it really.
Sorry for the long post, I felt quite happy when typing it though lol