It was simple science, really - science that you learnt about when you were young and ignorant and wow look the fire goes out when you put a glass over it - but it was, in all honesty, a summation of their relationship.
Fire. Wind. Bigger, stronger fire.
They relied on each other constantly, fueling each other, pushing them to grow, develop - and sometimes the fire got too big, and you couldn't put it out, and it wouldn't die down even in the still, unmoving air, even when it left the wind behind, the fire was still burning, moving, growing destroying -
Sasuke was the fire, of course - and you might be inclined to think otherwise, given Naruto's chakra situation, but there you are - and the wind was stronger than anything he'd ever felt. If this was wind (it really felt like a hurricane, or something similar), then everything else was a pitiful breeze.
The wind was too strong. The fire got too big, so huge and unstoppable that the wind supporting it was rendered redundant -
and then the wind was blowing through and empty, desolate land, full of no warmth and chasing after the sun - the closest warmth - on the horizon.
Wind can't really travel that quickly.
The sun sets.
Every day, the sun sets, another small fragment of hope is demolished and the wind still has to blow on, because it doesn't really know any other way to catch the sun, to regain the warmth of fire, other than try again tomorrow, try harder tomorrow, don't give up.
Nights are long.
Winter approaches and the nights get longer and colder, lonelier and more hopeless, and the wind blows on.
It tries again the next day.
The sun sets.