Cautious optimism is ideal in the process of becoming a physician. Blind optimism leads to poor choices and depression should things not pan out. We're not saying don't go for your dreams or that they are impossible, merely that you should be more realistic in your analysis of your situation, knowledge, and capabilities.
The premeds that I met Over the years that didn't make it were usually the blind optimist types. "I'm going to be a dermatologist." "I'm going to go to HMS." "I'm going to get a 40 on my MCAT." Kid 1 fails organic, retakes for a D, gives up. Kid 2 gives up in physics II, tiring of the constant churning out of labs while his friends are enjoying life. Kid 3 scores a 17 on their MCAT because they weren't able to function under the pressure of the exam, despite their As in every prereq and solid understanding of the material.
The ones I know that did the best were the ones who were always just a little bit unsure of themselves. The ones that put in an extra hour of studying at night because they never felt they knew things quite well enough. The ones that started off worrying if they'd even get into any med school, that worked their butts off and found they had top-tier stats at the end of the day and ended up with multiple acceptances in hand. When they'd stumble, they'd lower their expectations, and when they succeeded, they'd adjust accordingly. Idealism and optimism alone can't get you through this mess. You need a realistic eye, some serious perseverance, thick skin, and a good dose of skepticism to keep on the right track year after year.