Teach puppy to accept strangers' handling
My pup is 3.5months, starting recently she no longer likes strangers to pet her anymore. However, I want my dog to pass Canine good citizen once she grows up and the second item is: sit politely for petting. She would go sniff/chase/lay beside strangers no problems, but if they bend down to pet her especially suddenly, she would run back a bit and turn around to bark at them. She has no problems accept handling all over her body by me (I practiced lots of "grab-treat-release" to desensitize her), and she is fine if my friends suddenly move to pet her as well. She does not like vet handling her either, but she would give in after I hold her body. She would take treats from strangers, and sometimes allow them to pet her spine but not her head.
My trainer and I have a difference in opinions here. My trainer thinks I should let my pup grow and let her decide whether she wants to be petted or not. But I think I need to do something before she gets worse and I am thinking of teaching my pup to accept petting by letting strangers hand-feed her while they pet her, which my trainer thinks is counter-productive to engagement later on. I can see my trainer's point of view, but I don't like the idea of doing nothing and just hope she will turn out fine. I like aloofness in GSDs, but passing Canine Good Citizen is an important goal for me and also I am worried about her scaring little kids who pet dogs without permission once she gets older and bigger and becomes even more beautiful. Furthermore, I can't tell if this is fear/aggression/aloofness based, and I am worried about it turning into fear-biting when people try to pet without permission once she gets older.
What are your experiences with your aloof GSDs? How do they respond when strangers suddenly bend down to pet them at the bus stop without owners' permission? How did you teach your puppy to accept strangers' petting without hurting engagement and making it lose GSD aloofness?