When I first started reading, I was all, "But they're so little! And adorable! They're too cute to slash!" But one of the things I really truly love about the series (and Obata's art) is that they visibly grow up. Makes shipping them even easier, 'cause they're still just as obsessed with each other when they look almost like grownups.

Also, I hear that somewhere out there there's official artwork of them post-canon. I wish I could find it - I've been looking to no avail - but I hear they get even more slashable, which is a giant yay in my opinion.