Character Development > everything
Although, I think that Johnlock will happen (soon) it’s not important who he is talking to in that scene.That he feels - that Sherlock has feelings - is showing what this show is really about: love, realtionships and trust.
In series 1 Sherlock is an absolute amateur when it comes to social behaviors. He didn’t show any feelings or emotions (if he even had them). Understanding and respecting feelings from someone else (even John) was just impossible for him.
Eventually the people close to him taught him about emotions, feelings and human interaction. John played a major in that, but also Mr. Hudson and Molly were important. Without them Sherlock wouldn’t be who he is today. He wouldn’t have changed a bit; he would still be a lonely sociopath.
(whoops why do I always come back to Johnlock?! sorrynotsorry)
Now, he cares about his friends and he trusts John enough to show those feelings when they’re alone.
He understands emotions because he’s feeling the same.
That, everyone, is character development at it’s finest!