Why do dogs bark at some people and not at others?

Firstly dogs will bark to warn their owners that someone that does not belong at their home is present. Then if their owners know the people quite often they will bark in excitement and to say, hey if my owners know you then you need to know me to (basically to be centre of attention) and all of this is acceptable. It is when they continue to bark after a short period of time then this needs attention and training to say it is not acceptable.