If you all were sitting together on the couch then he would
see himself as an equal (or superior) member of the pack. Being
at an elevated level like a couch,chair or bed lifts his status in
the pack setting. In this situation, I would never let my dog get
away with growling at me, ever. He would immediately be made
to "off" whatever he was on & be put in a down/stay or sit. I
would also forbid any further laying on the furniture. Sounds like
Lefty would really benefit from the nothing's free treatment.

I first thought you meant Lefty was laying on the floor & someone
had bent over him where he could not see their face or hands &
that's what made him nervous.

My Buddy was a "give him an inch & he'd take a yard" type dog
also & I had to be very firm & consistant with him as he was
growing up. (Not mean, but firm, no nonsense commands)