Shuichi was well suited for the sun.
His hyper personality was a trait best enjoyed during the summer, when friends and family were bound for outdoor locations such as the beach, or the park. He was a lot like the sun itself, shining down on everyone around him and making them wonder what their lives were like before he appeared. He could beam through anyone's cloudy day and turn everything right side up again. It was like he absorbed all of his energy from the light…
Yuki, on the other hand, was well suited for the snow.
He hated the summer… it was always too hot for him. In the winter you could always put more on, but in the summer, it seemed that you could never take enough off. He felt most comfortable on dark days when his inspiration floated from the sky like the tiny flakes of ice outside his window. He would hole himself up for hours, feeding on the dark energy and thriving off the lack of sun… everyone around him felt miserable and depressed, questioning the existence of light in such a cold heart…
How odd that they would fall in love.