Talk clearly about your experiences to highlight your enthusiasm.
Instead of saying ‘I have always longed to be a dentist’, say ‘I have wanted to be a dentist since I was 11, after my experience of....’
If you don’t know how to express why you want to be a dentist, ask yourself ‘Why not?’
Why do you want to be a dentist and not a doctor? What is the difference between the roles? What does the first role provide that the second doesn’t?