Well, I think the big theory, at least from my perspective, is that first hearing about Sara's life may have spurred him. This is relevant, and I'm sorry if it offends the other ship, and I'll obviously edit if it's deemed to, but I think Pirates showed that when Grissom likes someone, he kind of puts up the fantasy wall.
When Sara kept proving to him, again and again, that she wasn't just the student or the tape-me-up blanket girl, I think in a strange way, he loved her more. He wanted to know more about her. In the other case, I think that wall came down and he realized that that person wasn't at all who he thought she was.
So Nesting Dolls kind of broke down the last of the support beams. This was Sara - with her broken past and hard life, which explained even if it didn't excuse, her outbursts.
He freaked out a little, which is understandable. He backed off.
And then Committed happened, and I think he reached a point where he went "At some point it won't even be about her waiting for me or not, it'll be about her physically being there or not" and then I think Grave Danger sealed that.
So while it's possible they got "together" after Committed, I don't think anything that constitutes as a full-blown relationship could've come before Grave Danger.
The utterance of "I want my guys back", him finally embracing and feeling that sense of family Catherine talked about way back in Burden of Proof...
I think it opened his eyes.
So the Butterflied speech came at a time when he'd distanced himself as much as possible, and as far as he knew, Sara was fine. It was an ignorant stance, but I think we saw part of that wall being caved even at the end of Bloodlines, him realizing that she had a problem and he didn't want to aknowledge it when he first saw warning signs.
He doesn't have to be responsible for her, but he certainly started taking her very much as an extension of himself - her blowing up at Catherine was about him as a boss, not her as an employee, he seemed to find no fault in her actions in the nursing station, and defended her... on and on.