You want to grab the reader's attention, but at the same time you don't want to make them roll their eyes. It's a fine line to walk.
Example #1:
"Ever since I was a young boy, I have wanted to be a neurosurgeon because..."
BORING. You've probably lost the reader already. This personal statement won't hurt you because the majority of the personal statements the PD reads are going to start like this.
Example #2:
"Brains. Everywhere. I look down at my hands. My fingertips have been replaced with 10 miniature brains, fully functioning and integrating into my neural network ever increasing my processing power. I then wake up with an enormous grin on my face. This is my recurring dream and it empowers me."
Ok. This opener no doubt captures the readers attention. With only 2 lines you have guaranteed that your personal statement will not be forgotten like the first example. Unfortunately it also makes you sound like a psychopath. There is a decent chance a resident will print it and put it up on their bulletin board next to funny patient quotes and consult pages.
Example #3:
It was 2 AM on a Wednesday and my second time being on call during my away rotation at <impressive university here>. We were in a crainotomy case when we got the page from the ED, and my resident asked me to go down and get started...
Well, this isn't exactly boring and it doesn't make you sound like a psychopath. The reader is interested and wants to know more. What was this case? How did you respond to it? What does this have to do with your desire to be a neurosurgeon. This is a good opener.
Example #4:
"I'm an actor. It's not brain surgery. If I do my job right, people won't ask for their money back." -Sean Connery
No. Resist the temptation to use quotes or references to pop culture. This is instant eye roll material. For some reason we all did things like this for college admission personal statements. I don't know why we were told to use the words and references of other people in what is supposed to be a personal statement.
TL;DR? Just write an opening that (1) is not boring, (2) does not make you seem like a psychopath, and (3) does not make people roll their eyes, and you will be good.