In my personal opinion, no matter the breed it's all in the way the dog was raised.

For example...
You could get a pit puppy from a well known dog fighter that has the meanest dogs in the city and raise it to be a therapy dog!!

It's all about socializing and training that puppy from the start.
We have (had RB Angus and RB Rocky) a few Rotties here on the board, and we also have a few pitties on the board and they all are sweet as pie!
Jada, Ebony and Diamond!! (along with others)

My sister used to live with a friend and they had 4 Rotties. I used to house sit for them when they went out of town and they all were VERY sweet!