Damons humanity...on or off?

It's been clearly stated through both words and through action that Damon turned off his humanity some point shortly after beginning his service to Sybil. We know he considered letting himself feel, but quickly realized that was a terrible idea...

That said, his actions since the winter premiere indicate that he has definitely flipped that switch back to on. In the past it's always been a chore and a half to get a vampire who has gone full sociopath to embrace his humanity again. And when they do it's an overwhelming experience.

Even by his standards, Damon has done some truly terrible *beep* over the past couple of months. If he truly is feeling again he would be unable to hide the impact of those things on a newly humanized psyche.

I guess it's just unclear and not inline with what we've seen in past seasons regarding the humanity switch. I guess the necklace triggered it somehow? Sybil flaunting it in front of him didn't affect him before why did it suddenly do so now?

On top of all this...not seeing ripper Stefan and unhinged Damon together for at last a couple of episodes before one has a crisis of conscience is a giant missed opportunity.