I've thought up this theory as to why Xerneas' and Yveltal's inactive forms are a tree and cocoon. As we all know, Xerneas represents Life, While Yveltal represents destruction, or death. And of course, trees are full of life. They grow with bright leaves, and the tree supports the lives for many other species. Yveltal represents the cocoon. Caterpillars eat leaves off of a tree, to grow and later, create a cocoon to protect them during their development stage. The caterpillar takes from the tree, and doesn't give anything in return. When someone takes something in life, they get nothing in return. The tree is being killed, by the caterpillar for personal gain. But without that caterpillar, the tree has no purpose. It just stands there, to do nothing. And without that tree, the caterpillar would die, it has nothing to feed on. So this means without death, life has no purpose, even though that tree will die, it creates life for the caterpillar, only to lead to it's death. So although life can make some happy, we all end up the same. Without life, death would be rendered useless. And if no one were to die, just like the caterpillar eating the leaves, life wouldn't have a duty to fulfill, and death wouldn't be able to survive. When that cocoon bursts open though, life is released, Life that will die. So life is only here to make you die, while death is waiting for you when life is gone.