To my way of thinking humans and animals are the proper subject of sculpture, as it has come down to us. Any other object can be an interesting object, in its own right, but it is not sculpture.
I am a human, my best friends and family and acquaintances are all humans so naturally I'm most interested in and moved by what is most familiar and known to me. I'm not blind to the grace and elegance of a piece of driftwood or a tree branch, but casting them in bronze does not a sculpture make. If everything is sculpture then nothing is sculpture.