Leaves falling from trees in autumn is so often equated with loneliness. Here EE Cummings puts the very action inside the word loneliness and the whole effect is one of sadness, silence and maybe even a tender sorrow.
But have you ever seen a leaf in the moment of falling from a tree? It seems more giddy to me than anything else, twirling to the ground in a kind of swoon, glad to be free. But I suppose this does happen most of the time with no one around to witness it, thus validating the loneliness comparison. Even this poem is lonely - look how it sits on the page with no words or punctuation around it for company, its vertical structure mimicking a lone leaf dropping to the ground, a quiet almost imperceptible event.
leaf - ee cummings