I take my consent from the dog. You’re its caretaker, friend, and family. None of that means the dog’s incompetent to decide and communicate desire to be touched.
edit: I should be more clear. I ask for permission from the owner. I say “Can I say hi to your dog?”
Then I say hi by putting my hand out to smell. Then if the dog wants a pet, I’ll pet the dog.
Permission to approach from the owner, but consent to pet from the dog.