I really wanted to be a vet when I was in elementary school/middle school. Kind of changed my mind to physician or something in human healthcare in H.S. b/c I thought it was more practical (way harder to get into vet school and pays much less afterwards). Was pre-med throughout college but really made the final decision sometime in sophomore/junior year when I started working with physicians. I was 100% positive I wanted to go into ortho or sports med.
That was 8 years ago, and I was still solidly set on some form of sports med until about 3 months ago when I fell in love with psychiatry after doing my 3rd year rotation (which was pretty shocking b/c my dad has a PhD in neuropsych and I hated neuro).
Something I tell everyone who hasn't gotten as far as me is that it's great to have a goal/passion/dream, but keep an open mind and don't shut any doors before you know what's behind them.