I think it would be OK for someone to check grammatical errors, etc. for you......but your personal experiences about why you want to go into dentistry is really up to you.
Make a list of things you love about dentistry. Do you love working with people, love the artistry, etc.? How did you come to your decision? Have you had positive experiences with dentistry in the past? Also talk about who you are and what you want from life......how dentistry ties into this.
Someone I know who applied last year was having a really hard time writing hers. So her mom asked her questions about why she chose dentistry, and she jotted them down, and from there, she could formulate her ideas that she had verbalized into an essay.