Because this is how Frisk always is.
Undyne's chasing them? Same expression. Papyrus takes them out on a date? Same expression. Mettaton tries to murder them? Same expression. About to kill a monster? Same expression.
I got bored. ;u;
Undertale (c) Toby Fox
Blank Meme: http://reimaniquis.deviantart.com/art/HTF-20-Facial-Expressions-Meme-201613001
NOTE: I am no longer using this account, however I will keep this picture up because apparently people seem to like it, given it has 3400+ views.
Link
Deimos-AC
You should change it from "Frisk" to "Andrew". ;u;