My pre-health advisers emphasized that the personal statement is not the place to restate your resume. In general, it's an essay about your growth and how you know dentistry is a good fit for you and vice versa. This is a chance to show the admissions committee your personality before they meet you face to face. Personally, I recounted an incident that happened when I was young that first got me interested in dentistry, talked about certain hobbies that showed manual dexterity, and spent the majority of it talking about a singular, profound event I experienced at the dental office I worked at that tied back to my childhood experience.
Some ideas to get you thinking are: Is there something very memorable about your shadowing experience you want to share? Were there any critical points in your life when you thought "this is why I want to pursue dentistry"? When was the first time you considered dentistry as a career, where were you and was it significant or just a small blip in your memory?
Hope this helps!