Oh I don't know if I'll be able to explain this properly. When you sleep, your body heat warms up the top of your pillow and it can be annoying to sleep on a warm pillow. So you flip it over to find the cold spot underneath. Wishing that both sides are cold means you never have to search for a cool spot. You get maximum comfort all night long.
Long story short, it's a complimentary phrase as a thank you.

I NEED THE NOVEL LINK Y'ALL. ALSO WHAT CHAPTER IS THIS IN THE NOVEL