I have a 7 month old male lab and a 5yr old male lab. I don't have the problems you have but to me it sounds like the dog is jealous. My boys sometimes argue as to who can get closest to me as I am the pack leader but they don't have a problem with my partner.
Saying that when we first got the oldest one I didn't live with my boyfriend and whenever he came round Harvey (dog) would always try and sit between us, he never got agressive but was definitely trying to push my partner down the pecking order. A few deliberate pushes off the sofa (where he loved to sit) and being ignored soon made him understand it was not allowed and he had to share me.
Are there any particular occasions when the dog gets aggressive with you? I am presuming it is when you are both showing affection for each other and ignorning the dog.