sans peace is found in silence. Where he gets to think and feel. Where he can be weak and cry, not having to put up a front to make a brother whom he loves to the end of the world and back, that way he won't be sad, he won't feel down and feel what he feels. That way he won't cry once all hope leaves sans' soul, the only way it's stuck together anymore.
Papyrus' happiness is found in the form of noise. People talking and having fun around him makes him smile. Having his brother sans happy makes him happy. He watches his brother's life energy decrease each day. His hidden powers being used behind everyone's back. Powers that he hates, and he loves at the same time. He never wants to use them, but has to. To make sure his brother won't parish before him.
Toriel's kindness is found in children. She takes them in whenever she can. She'll help anyone out to make them smile, reminding her of the ones she lost. She wonders if she stayed with Asgore if she would have had another child. She decides on yes, she would have had one. But she would have cried during their birth, cried when they started school, and wept at every time they didn't need her. She would be reminded day and night that no matter how much she prayed, Asriel and Chara would never return. They were dead. And will be forever.
Asgore's happiness comes from protection. Willing the royal guard to protect monsters and kill humans. Protecting himself from remembering the children he lost, the wife he loved, and the friend that he sealed his doom for. Hoping for Toriel to come back to him if he spared a single person. Never knowing where she went, or even if she lived another day.
Flowey's happiness came from everyone. He loved everyone with all his soul. He didn't care if you were evil or good, monster or human. He only cared if you loved him back. But after getting used and taking his friends soul. He doesn't anymore. He hates them all. He wants to kill them all. He wants them to kill him. He wants his soul back, he wants his friend back more.
Chara was never happy. Her chocolate only made her smile, never gave her cheer. Her friend was a stupid crybaby and could die for all she cared, she only need him for her plan to work, which it didn't. Now she depends on another crybaby to get her through her needs. But even he doesn't do what she says. He cries about killing too, and it makes Chara…sad? She doesn't know but she hates seeing him sad, and wants him to stop crying. She never wants to hurt him, but why?
Frisk is happy. He is happy with Chara, with Asgore, with Asriel and flowey, with Papyrus and sans, with anyone he is with. He never wants to hurt anyone. But with Chara he will.
Frisk is always happy.