What Hoover has said is a good point (not saying it applies in your case as we can only go on the way you've described it, but it's a possibility). Dogs do need some sort of boundaries so they know where they are - its a bit like the child who is constantly pandered to, given everything it wants and is the always centre of attention, 9 times out of 10 it will become a "nasty little brat" with no respect for its elders.
I would agree that a behaviourist would be the best course of action however, you can't really cure it until you know the cause. Best of luck!