Agression when people leave the house

We rescued our abandoned 1 1/2 yr old JRT about 8 months ago from a shelter. His behavior and integration into this two person household has been remarkable. Here's the problem -- He hates it when we leave the house but becomes really aggressive when visiting family or friends make towards the door. He goes from being friendly and playful to excited and vicious when any one makes an overture towards leaving. He also does this when either one of us leaves the house to run an errand or leave for work. Any thoughts as to why he does this and how to best handle it. He is otherwise a wonderful companion.