In med school, I was taught to address patients formally. Mr or Mrs or Doctor or Sir or Ma'am or Father or Sister or whatever. It's a matter of establishing professional boundaries. I'm not there to make friends with my patients. I'm there to care for them, and that should be clear.
Sometimes, patients like to be called by first name, but for the most part, people appreciate the respect. But with some patients, things change. On my terminally ill patients, I can't help after a few weeks to start calling them by their first names. I've been seeing them for so long, and I've met all their family and friends, their spouses and children, their parents and best friends and siblings. I've talked to them all about matters of life and death.
It seems to me that after all that, I should be calling that person by first name, and they can call me by first name too, because you can't be that intimate and still stand on formalities.