I've been pretty good at science (especially chemistry) by the time I was a freshman at college, but it was volunteering as an undergraduate research assistant and presenting my work that made me think, I want to do this for the rest of my life. I loved the process, from finding an interesting problem, to mapping out and performing experiments, to collecting data, and going in surprising directions. Frankly, the lab is where I felt most at home through all of college.
I don't really have teaching experience other than short presentations with elementary schoolchildren, but it's rewarding to see that "click" within people as they realize that this science stuff matters to everyday life, and it's so freaking cool! This kind of realization is to me especially important at the collegiate level because the curriculum is most suited for teaching how to use the scientific method and critical thinking in the context of chemistry. Even if none of my students become scientists in the "pure" sense, a deep understanding of how to interrogate the world and communicate to other people about those findings is important to future doctors, nurses, politicians, teachers...heck, everybody, in my view. /soapbox
I'm gunning for the PhD partly because it's the most direct path toward becoming an independent researcher in my field, partly because I might as well dream big or go home