The Tucker Files: Golden Retriever Aggression?

I have to admit, Tucker has been more of a challenge than my previous dogs (except for a male alaskan malamute)  He is very dominant and I am pretty good at keeping him in check.  He has never shown aggression meeting another dog, and has never growled or snapped.  I can take a raw beef bone from his hands and stick my hand in his food bowl.  He hasn’t had a lot of exposure to young children, and when he did he approached them with a little too much “zest”

I managed to capture Tucker making his “killer face” tonight. (bared teeth, crinkled up nose)  At first I thought it was cute and was trying to teach him to smile, but as you can see, he actually sort of snapped at my finger, even though he didn’t bite hard.

On our last visit to the vet, when the vet reached for Tucker’s head, he made this face.  Needless to say, the vet immediately went for the muzzle.  Tucker took it well, and it was removed before the end of the exam.  I told him Tucker sometimes makes this face while we are playing.  The vet set he couldn’t take the risk.  I told him I didn’t blame him one bit.  I think it had something to do with the high strung vet tech who tried to weigh Tucker and backed him in a corner and he peed on the scale.  She continued to try and force him on the scale and you could tell he was avoiding the pee and not wanting to step in it.  I eagerly offered to clean up the pee and told her he wouldn’t get on the scale like that.  She shoved us into the exam room and told me we were “holding up progress”  This made me both irate and embarrassed, and our visit had not started off on a good note.  I’m not making excuses, but the place was literally packed and I think it was a bit much, even for Tucker.  I ended up crying on the way home and thinking that I was a failure in terms of having an obedient, well trained dog.  (When I have a treat in my hand Tucker is actually extremely smart and obedient!)

I am curious as to your thoughts golden experts!  Is this a sign of aggression?  If so, any suggestions on correcting this behavior?