I didn't clue in on that he was nailing trees to the side of a mountain on first viewing. Why in this cartoon land does gravity exist on both sides of a right angle?! With that momentary bewilderment removed on second viewing, I could focus on the very touching plot. Tear jerking stuff indeed; I'd compare it's impact with the final scene in one flew over the cuckoo's nest and this terribly sad korean movie I once watched where the protagonist drowns himself when the town he's lived in all his life is relocated due to the construction of dam. A girl and heartbreak was also involved there, but I've purposefully forgotton the details as it was the saddest thing I've ever watched. Oh, and this short film I saw on that defunct CBC show Zed, where after being shot as an innocent bystander in a bank robbery, an english professor's last thoughts are about the perfect beauty of a phrase used by one of his childhood chums in a baseball game, the prompting for the discovery of his love for the english language.
I don't agree with your analysis. Kiwi thingy doesn't have wings capable of flight. His sole dream is to fly; the one thing he can't do he yearns for the most, and so he divises and executes an elaborate scheme to achieve that compelling dream, if only for a few brief moments. There's a catch however: It will kill him. He knows this and commits suicide anyways, for in those brief moments before he dies he will realize his dream and live. It's about a single compelling dream that the protagonist chooses to realize at the expense of his life.
That being said, just as beauty is in the eye of the beholder, so it is with something like this.