dogs are not always nice

If you see a dog on the street and its siting down,you shouldn't go up to it or even try petting it if the owner is in a store where dogs are not allowed,the only thing you could do is ask the owner because you don't no what kind of dog is,and how it reacts if it doesn't have a owner pass right by it or around because some dogs,when they see you they would go crazy and bark out loud and try to attack you,in that they would bite you. Has this ever happen to you?Or part of this happened?