In every season, the leaves had reason to hope their hang upon. In the autumn... just hope.
It's only in the fall that they lose their amber luster. That they may lose their shine entirely.
We know, as we are, that their death is but an illusion of something better.
But we can't tell the poor leaf that it's purpose is to die. After a life well served, it will be simply discarded. We can't tell this to the leaf, because it won't understand.
Having done it's duty, it will dutifully lie in the dirt to be consumed. That is its mission.