My old Black Lab Duke used to do the same thing. He lived two weeks past his 14th birthday and died in my arms. I still miss him
Labs are just THE BEST!
My lab would grin like that also. And no it was not being mad, defensive or scared. I thought it was something it learned but maybe labs do grin at times. Now if it was scared or defensive it’s hackles were up and it was barking. Never did that to me or those it knew but only the strangers. I loved my lab. Also died at age 14.