What a long, sad trip it's been

I've been wondering the same for so long. I don't know the answer. I keep thinking back to "Born Under a Bad Sign", which was a stellar performance, and the first time he got to take Sam down and dirty. He went full-on with his performance then which was alright because it wasn't Sam who was acting that way. Then there was AHBL Part 2, when he shot Jake there at the end. He didn't affect any look of sympathy there at all, just...pissed off vengeance. Which, again, it was okay, because we'd just been told he might have come back wrong and while that is old news by now, it wasn't then. But then there's season four. In "Lucifer Rising" he was directed to go the whole "I wish I didn't have to do this" martyr route, which rang so incredibly false to me, as it was the first time in the entire season he showed any regret. Which means...he wasn't specifically directed to show regret and sympathy up until that episode, but that doesn't mean he couldn't have done something himself with the material he was given. Jensen does it all the time! I think it's because he's so fan of playing evil!Sam, and the problem with that is that the writers like playing chicken with evil!Sam which never, ever leads anywhere.

tl;dr of saying, I think it's fair to blame both Jared and the writers. Personally, I think Jared's acting took a nose-dive after season four and in season six every time he was on screen all I could focus on was how bad an actor he really is. He can do pissy and evil really well, but emotions on the sympathetic spectrum? He lost that somewhere along the way.