Smile. A smile puts people at ease, satisfies them that you're paying attention, that you have some sort of understanding, that you like them, that you care, that they're talking to a living person and not a blank wall. Smile. A smile lets them assume that you agree with what they're saying; you don't have to say anything, answer, commit yourself to anything you'll feel like taking back later. Smile. It's easy if you practice every day. It's just tightening up a few muscles in your face. It looks creepy if you only smile with your mouth, you should try to get your eyes into it too. They should narrow just a little bit as the corners of your lips force your cheeks upwards. Maybe try closing your eyes as you do it, what they call a 'fox-smile,' it looks playful. People like playful.
Goku always returned smiles with happy, honest grins. Seeing that look made smiling back feel easier, almost natural rather than a distortion. Goku made smiling easy, just with the way he was, and Hakkai knew that Goku appreciated and craved smiles. He didn't need to bother when Goku was elsewhere, or asleep, because he was quite sure neither Gojyo or Sanzo gave his smile any notice. They knew it didn't mean anything. He'd practiced it so well that he could do it all the time, he didn't even have to think about it anymore.
Kanan had always smiled when she looked at him. It was warm and soft, like being held in her arms. She had told her brother that the world was better if he smiled, and so he had started practicing then. After Kanan was taken from him, after he'd punished the ones who had taken her, when he was looking at himself in Gojyo's bathroom mirror, he'd thought that maybe he should start practicing again. Because Kanan had said that it was better to smile.
Early on, Gojyo called it creepy, but it didn't seem to bother him anymore, so long as Hakkai was directing it at someone else. He took offence if the smile was turned on him though, it was the same as someone lying to his face. Sanzo didn't care one way or another. He thought it was stupid; he didn't see the point in putting people at ease. He felt no need to please anyone. And Hakkai supposed that he had felt the same before Kanan had asked him to smile.
His expression before then had been dour and elderly. Like the one Sanzo wore now. It was easy to recognize that expression in Sanzo, he'd recognized it very quickly after meeting the man. Reactive Attachment Disorder. Common to orphans. From being emotionally neglected at a crucial stage of infant mental development. The result being a child and later adult who's world ended at the epidermis; everything beyond their own skin is someone else's problem. In their own minds, they are emotionally self-sufficient.
Kanan had shattered that belief, and Hakkai had never been able to repair it in the wake of her passing.
Maybe that was Gojyo's fault. Perhaps the cure for a withdrawn personality is to team it with a high-maintenance one. One ego damaged from not having a mother, one ego damaged from having one. Gojyo pressed for feedback, demanded opinions, instigated arguments with every breath. He forced Hakkai to listen and react to the outside world. And Hakkai was the only 'woman' Gojyo didn't see his mother in.
Maybe it was Goku's fault, for taking smiles at face-value. He trusted implicitly, like a child who had never been hurt or lied to. The world did not end at his doorstep. A doorstep was all he could see for five hundred years, and yet he had never stopped believing that there was something beyond it. His spirit went beyond unsinkable, and he could make an artificial smile real through the sheer force of his belief.
Or maybe it was Sanzo's fault, for looking so much like the mirror where Hakkai had first started practicing. Maybe because looking in the mirror gave Hakkai something he wanted to leave behind. Because after the artificial walls he'd erected in his childhood were gone, after Kanan had tumbled them and then left him exposed to the elements, looking at someone else's walls, armored and reinforced against invasion, made him shudder at the thought of going back to such a solitary world.