When I had my rottweiler she was friendly as heck! I took her everywhere with me. TSC, pet stores, the race track the laundry mat you name it.
My landlord had a key and walked into our house while we were sleeping, I heard her growling (she knew this landlord and never had an issue with her before) and she got more and more intense the closer the landlord got to her. We happened to be asleep upstairs. I waited (I know bad on my part but I was curious and hoping she would just leave!) once I heard Belle get really loud growling I got up and called out she stopped and was fine after that. Another time My other half's brother tried to sneak up on us and jump on us while we were asleep (again she knew his brother and never had an issue) she started with the growling and got louder and louder the closer he got to us. I was watching him the whole time trying to be sneaky with and whatnot. He went to jump on us and the dog jumped out at him. He just about pee'd himself. I told him to stop early on but he didn't. Guess he learned the hard way!
I let me friends Grandmother have her, as she needed a guard dog (though I still didn't think she was a guard dog) and was very lonely and lived alone. Belle really took to her so I felt she was happier with her than with me. The Grandmother had went on vacation for a few days and the daughter who Belle knew very well was taking care of her. Belle would not let her in the house for anything! But when her family was around she was the sweetest dog! Wouldn't hurt a fly. Loved to be around people, loves getting belly rubs!
Miss that dog like crazy! Though I know she does not miss moving around and living in apartments. And enjoys guarding her own home now.