Mork and Responsibilities
Imagine someone who just existed. Things used to happen to them and they brushed off the stuff that required real effort. Then one day, it led to devastating consequences and they concluded that their lack of responsibility was at fault.
Mork is someone who learns how things work so that he can fix them. However, even though it looked like he was the closest person to his sister, he wasn't there for her when she needed him.
He spent a year and a half alone with his pain, grieving and trying to ignore the guilt that consumed him to be able to breathe. He can't express his guilt because he doesn't think he deserves to be comforted. He turns his grief and guilt into anger.
People gave up on him because of his past mistakes, deciding that he was not worthy of trust, and seeing him as a monster. He still had Porjai, but he put a distance between them because he didn't want to drag her down with him. He was afraid to become the monster in her story as well. He didn't want to be responsible for her misery.
And then he meets this person who is lost and lonely and hurt and bitter, just like himself. Day gave up on himself because people gave up on him. Mork can relate to that and he finds himself wanting to fix him.
He dedicates himself to this mission wholeheartedly, as there's nothing that he wants more than to fix things- to fix his past mistakes, to conquer pain, to be worthy of trust.
He can help, he can make a difference. He's finally doing something with his life that feels meaningful and important which must be fulfilling and quite addictive to him. Cathartic even.
This person starts trusting the world through him (as per @heretherebedork), relying on him, and refusing to see him through the lens of his past. Day gives him a chance to set things straight, to be better, to do better and Mork embraces this responsibility.
Ultimately, what scares Mork is not that Day might choose someone else instead of him. What scares him most is letting Day down.
