"Even angels have bad days," she told him once, and he kept that memory of her words and her soft smile in his heart. He could almost visualize it, a scrap of paper with her looping handwriting and bright pink ink, a little heart at the end, rolled up tight and furled inside – a locket, perhaps, or a shell. A seashell with its own creamy pink heart, protecting her with ridges and whorls from whatever might strike at him. He remembered that, in the days after, riding alongside Dean and Meg and headed towards what might well be his own destruction. It's not just the words, it's what they meant: he didn't have to be perfect. He was allowed to be scared, for himself, for her, for the hopes he had for their life together.
And when he came back he kept thinking it, as he fumbled back to her and to what they had, when her patience ran out and his own clumsy heart couldn't take it. When she had a terrible day and burned a batch of pies at work and got a flat tire and came home late and he hadn't made supper, when her lips flattened out and her voice went cold and sharp as the metal she worked with, he remembered that days like this happened to everyone. That if he wanted to keep her with him he'd have to remember that she wasn't perfect either, that for all that she held him together and kept him breathing she was only human. Wondrously human, in all the best ways. But not possessed of infinite patience.
He went upstairs that night and ran the bath and poured her a drink. Tried to remember what else he could do to make her smile and take the tightness out of her face and shoulders. Other than the obvious, anyway. He pondered the ring that was hidden with his things, the reminders of his old life, and decided it wasn't the right night to take it out and ask her an important question. Instead he sat beside the tub and held her hand, and told her that even angels had bad days, and he didn't so much as frown when she splashed him. In fact, he decided that since his clothes were already wet, he might as well join her in the tub.
Later he would realize this was not the wisest decision he had ever made. But he was certain it was one of the best.