I spent about 50 hours shadowing a non-family member dentist.
50 doesn't sound a lot, but I used to go out to lunch with him like everytime I was there. We got to know each other on a [kinda] personal level
When the time came for him to write the letter for me, he was so nice, he actually called me on the phone and told me he was working on my letter. He started asking me some personal questions like where did I graduate from, what kinda grades do I have, what was software engineering like, ya know the same kinda stuff we used to talk about when we hung out during lunch hour....also personal information about my life in general.... I am assuming he needed that stuff to make the letter sound more personal
I think every dentist is different. One of my friends shadowed a physician who basically told him to write the letter and the doctor would have no problem reading it then signing it. Everyone is different I guess