A fear of strangers —
Firstly, you do not have to know the exact route cause as to why your dog is fearful of strangers, to be able to help them.
The first step is very specifically, identify all those your dog is afraid around – who and when.
Then, for the early stages of the training process it is all about lowering your dog’s (and your own) stress levels, by avoiding these triggers, to help them become amenable to teaching.
From here, starting outside of the reaction zone (the proximity where your dog is likely to react to strangers) ask for commands and ‘tricks’ to be performed in the presence of the stranger. The commands could start with ‘Watch’ (watch my eyes). Gradually over time move forward in the invisible scary realm (this varies from days to weeks) showering your dog with praise, affection and some treats (vary these and make them great!) if very little to no reaction is taking place by your companion.
When out and about and you come across a stranger, praise heartily and reward any and all instances of favorable behaviour by your dog, in the strangers presence (perhaps associating with a word) Selina McIntyre, Dog Behavioural Consultant & Dog Trainer, Dogs Best Friend.