Benedict has known Una Stubbs (Mrs. Hudson) his entire life because she is a close friend of his mother. So the affection Sherlock shows Mrs. H is completely real and completely Benedict's doing (Moffat didn't write it in the script initially).
that awkward moment when you realize that you watch 4 out of 5 of the shows that make you a psychopath. then there's the Sherlock side of you saying, 'I'm not a psychopath, I'm a highly functioning sociopath.'<<my thoughts exactly