I arrived at my decision relatively late - like, last half of undergrad late. I originally wanted to be a PA, but then I spent an entire summer doing job shadowing and I realized that I didn't care enough about the people I would be treating. I had always loved being in the lab - any lab - where I could connect concepts with observations. It always helped to give me that "click" of understanding.
I chose biochemistry so that I could potentially get into drug research, which I figured would be a good way to still address my interest in health care without all the direct contact with patients. I decided on a PhD program because I'd like to have the opportunity to be a leader in the lab and help make decisions on the direction of the research.
Also, if I'm accepted I'll be the first person in my family with a doctorate of any kind. I think that would be pretty cool.