Well, I might not be the one who is suitable to answer the question, because I failed cards fellowship some times. But I think your question of 'why cardioogy' is very fair one to ask, and I was surprised at seeing only a couple of relevant answers so far. Maybe everyone else is in the post-match residency heaven and does not need this forum anymore. (Don't be upset, that's the life, and another wave of innocently passionate people will hit here around this summer) Note that mine would be also not relevant, but this is kinda story that I have, should you want to know.
In my case, I liked physics VERY much in high school. Thought about doing physics forever, but human body was like a pinnacle of physics to me (don't ask me why I thought like that). When I came to med school, I thought about CT surgery or Ortho (!) for my future career just because those fit into the 'physics dream' that I had in the realm of medicine. Then I wanted to be a cardiologist since MS2, when I learned physiology and pathology. I taught CV physiology for med school juniors as a MS3. As a MS4, I liked Medicine more than surgery because medicine people actually think more and logical than CTS friends. I knew that I gotta be a good Medicine doc first to be a good cardiologist, and focused on studying medicine. So I was pretty much determined from the beginning. I wanted to be a good physician-scientist in cardiovascular disease.
But why failed repeatedly? Because I am a foreign medical graduate from good (but not top-notched) IM residency program. (Again, that's the life. I accept legitimate discrimination.) When I came to the US for training, I found out the cards make good money and relatively nice lifestyle, and hence very competitive even among the US grads. But I know many of my friends would have their own genuine reasons to choose that path. Even, I think it is reasonable to follow money and lifestyle. It's your life, and following whatever reason that fits you will be totally fine.
I was undaunted up until last year, but now I am not sure. I am thinking of going back to my country, or going somewhere else to be a cardiologist because no programs in the United States would accept me. It is very sad given my long list of publications and academic achievements during and after my residency training. Well, I came to the US because I wanted to be a cardiologist, and now thinking of leaving here because I still want to be a good cardiologist.