Toddlers are willing to try anything. Why not have cereal with orange juice instead of milk? Why not run at top speed down a flight of stairs? Why not stick toys up your nose to see how it feels?
It seems that toddlers follow a mantra - "let's just see what happens" - not from free will but because it is one of the most basic human dispositions. It's baked into our DNA. As children, it's how we learn about the world. Even as adults, we learn best through making our own mistakes and absorbing those lessons for the next time.
Except, of course, when there is no next time. Humans have an unfortunate tendency to try things that could get us killed. Did you hear about the man who tried to connect the conductor on a nearby electricity pole with his house using jumper cables? The jumper cables were designed for 12 volts, but the conductor sent 7200 volts, not just through the jumper cables but also through the man's body and into the ground. The Darwin Awards are littered with cautionary tales of "just seeing what happens."
Obviously, applying to medical school won't get you killed (but going through medical school might). But if you apply before you're completely ready, you may indeed harm your chances of ever getting into medical school.
So, we hope we've made our point that you should only apply to medical school once. But that begs the question - how do you know when you're ready to apply?