Haven’t posted here forever either but thought I would pop in and share my TLK/curse theory as well.
I think that both people have to be fully committed to being against the curse and 100% invested in the other person. If someone doesn’t have their memory than they can’t be 100% invested in the other person. Oddly, I actually think BOTH these things are the problem right now. Just like we saw when Belle first kissed Rumple, the Dark One power embraces the curse. So even though EMMA really does love Killian, because a part of her is being controlled by the Dark One power she can’t be fully committed to being against the curse even when SHE wants to be. And for Killian, even though he really does love Emma, because she is currently the reigning Dark One (and as we saw he has chosen NOT to give into her when she is being controlled by the Dark One) he cannot be 100% invested in who Emma currently is right now. That doesn’t mean he doesn’t love Emma Swan as much as he always has and isn’t willing to fight for her as much as he always has been. It just means that when he kisses Emma now, he knows that she is not really HIS Emma at the moment so he can’t be 100% invested in her – hence the reason he was able to walk out the door when she invited him to stay at her house. It makes him all the more aware that he needs to find a way to destroy the Dark One power and save HIS Emma.