>>536972031
I get the feeling that Deltarune is a sequel to Sans’ and Papyrus’ journey in Undertale.
So Sans would have a reason to be fond of Toriel’s company and desire to save her in chapter 4. Sans would also overcome “I will never see them again” after his experiences with being around lightners who are like those in his original world, all except Susie, Kris, and Ralsei. Thus, even if though they’re not the FUN gang he had back then, they’re still the FUN gang he desperately wanted to save back then.

And Papyrus would be trying to figure out who he really is after whatever you do to him in Undertale, and try to be a better knight than he originally was with Undyne’s advice.

Think of it this way.
SANs had photos of the previous world he explored (which wasn’t Deltarune or Undertale), then in pacifist he is in a community with the Undertale cast, but in the goner event with ‘Suzy’ he pulls out the “Don’t Forget” picture of Susie, Ralsei, and Kris. So, he’s planning on leaving Undertale to Deltarune.