From my experience, the one thing that motivated me was that every time I study about a certain disease, I know it will eventually help others. I have had medical exposure early and thus my journey of medicine began when I was young. My motivation will not be directly applicable to others but in my opinion, motivation/goal being especially personal is what made me focused, to which in this particular case is to help those who have not had any proper medical attention. The amount of information that you have to study is daunting for sure, but it helps tremendously once you have the experience for it. As years go by, you will develop new motivation and obstacles; maybe I might want to switch specialties; but in the long run, at least for me, it's worth it!
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From @uglyduckly