This last line of the story communicates how close Buddy feels to Sook. When he hears of her death, he says it is as if some irreplaceable part of himself had died.
Sook made Buddy kites for Christmas every year, and he also made her kites. One of the happiest days in his memory is a day they flew their kites together after Christmas. Buddy describes that sunny day, in which he and Sook sprawled and watched their kites "cavort." Buddy says he is as happy that day as if they had won the Grand Prize in their coffee-naming contest.
Sook is also very happy, and says,
As for me, I could leave the world [die] with today in my eyes.
When Sook dies, Buddy searches the sky, half expecting to find two kites heading together to heaven. In doing so, he is thinking of the happy day they spent flying kites and about the bond he and Sook share. He feels as he looks at the sky that she is taking a part of him to heaven with her. He knows she can never be replaced.