Why do dogs bark at strangers??

I can't speak for all dogs, but one I had growing up as a kid, well, she barked and barked at strangers til she was allowed to go play with them. Not a good guard dog, but a fun friendly member of the family. I miss her.

because they are strangers and its new face for them.

I believe it's because they are strangers. Dogs don't usually like people they don't know, so they bark as a way of protection. My dog usually barks at strangers because he doesn't like people invading his own space. Dogs hate if someone strange invades their space.

My dog barks at people who are at a distance..when they come closer and she can see them better she starts to wag her tail. She is leary of people who are on bicycles, pushing strollers or other wheeled stuff. I guess just like people dogs can be leary of strangers until they get to know them better.