So Dream Sans's theme is called Wistfully right? WELL, wistful means "regretful longing" (DREAM WHAT DID YOU DO?), and i think i've figured it out-

Dream doesn't want to hurt other beings, especially not his brother, so maybe the "regretful" part is: he hurt his brother, and the longing part is: Dream wants to see NM again, not knowing if he's okay

;-;

L O G I C

I'm getting a lot of agreement and positive commenting-
Thank you uwu