OP was interested in surgery. It seemed like he was disappointed because he was interested in some subspecialties that are very competitive. I am not fear-mongering. For people like the one we are talking about, that mistake does significantly change the equation. This thread is talking about someone who was interested in surgery. There are several surgical subspecialties for which step 1 is damn near "make-or-break."
The reason why it's unfortunate that this is the way medicine works is that in most other fields, even if there is a high barrier to entry, you will not get bottlenecked into a career that is substantially different than the one you intended to enter. To make it more clear, I'll use this example: I want to be a psychiatrist. I have for a very long time and it was the reason I went to medical school. Imagine a world where psych was one of the most competitive fields. People who were in love with psychiatry and passionate about it might well choose something other than medical school if they couldn't see themselves being happy as a medicine doc. By the time you get to the point that you're bottlenecked, you have too much invested for it to really be feasible to go do something else you might be almost as passionate about. The broader implications of this type of system are that med school will attract fewer people with focused passions and more people with passing interests who would be okay with any specialty. I think it's hard to deny that there's social utility in having people with focused passions in the fields they're passionate about, but whatever.
Besides complaining about unfairness being a necessary part of exposing aspects of a system that might need to change, it also serves a function even if nothing does change. At least by recognizing that a situation is not completely fair (even if it may be the best system we have) we do not allow ourselves to just normalize these types of situations. If we lived in a situation where it was inevitable that everyone would develop a cancer and we all had the same level of care (the same "shot" at life, if you will), s0me people would live and some people would die but at least they'd die in the fairest way possible. Just because this situation is inevitable and it's "the best we can do given the circumstances" doesn't make the situation okay. It's still tragic, unfortunate, and worthy of lament.
For various reasons, I think it's important for people to realize this instead of just getting trapped in the learned helplessness of "oh, well this situation sucks but there's no way around it so I have no right to be upset." I think that's a long-term recipe for emotional disaster.