Your personal statement should be used to do 4 things:
#1: Answer this question "Why dentistry?"
#2: Clarify any obscurities in your resume. For example, I had 8 W's in my freshman year of college so I explained what was going on in my life at the time in my personal statement.
#3: Describe anything about you that's unique that isnt so obvious in your resume. Can talk about family.
#4: Allow the admissions committee to FEEL your commitment to dentistry... the "fire in your belly".
Hope this helps.