The title and question are two very different answers for me.
What made me want to be a psychiatrist over other specialties? Interesting pathology, endless research potential, the importance of clinical judgment for diagnosis and treatment, significant flexibility in treatment setting and area of specialty, lifestyle/"normal" work schedule, need for more MH care/job security, earning potential, all the crazy patient experiences, and just being generally happy on all my psych rotations.
What was the moment that made me fall in love with psychiatry? I've had several and continue to have them, but the initial moment was actually the first psych patient I ever saw. Had a kid in his early 20's who was admitted for "psychosis" and was fairly aggressive on admission. The only reason he came in was that his GF said that he was acting weird and that she was really worried for him. I connected really well with him as we had a lot in common in terms of background and life experiences. I actually got close enough that I kind of became his tether to reality and he trusted that what I said was the truth. Example: his dad came to visit and the next day the kid was convinced the whole visit was all just AVH and that his dad was never really there. Didn't believe anyone until I told him his dad really did come and that I'd met him. Obviously, he was started on meds immediately and they were extremely effective. Two weeks later he was almost completely back to his normal self; it absolutely blew my mind. Before that rotation, I had a completely different view of what psychiatry was and was 99% sure I'd never do it. I kept an open mind for the rest of my M3 year, but nothing else came close to that rotation for me.
I still find it almost magical when someone comes in completely manic or bat-s*** crazy and by the end of their stay you're having calm, lucid conversations with them.