We form thoughts separate from our current circumstances likely thanks to a cluster of brain regions called the default mode network, which lights up with activity when we fantasize. It constantly interacts with other bits of the noggin, including those more heavily involved in mood, says Aaron Kucyi, a cognitive neuroscientist at Northeastern University. That makes our daydreams deeply intertwined with our emotions, hence why on duller days we might imagine more blissful scenes.
