I don’t know if anyone’s talked about this, but I want to combine two theories I’ve read recently.
In the mind palace, we see a completely different John. He’s confident, focused, completely ready to face Sherlock when his eyes turn to him. But the palace snaps away, and in reality, John can’t face him.
Does Sherlock wish to see a more confident John? Or possibly, does he wish to see a John that won’t turn away, that will accept him head-on? A John who isn’t insecure about his feelings towards him? A John that won’t shut him out anymore because he died, or because John’s proposed to Mary, or any other excuse to wander off-track and not address how he feels.