A theory I have been kicking around is that Middle Earth is a fantastical representation of the conscious mind (perhaps Tolkien's own?) being driven by the destructive force of an inner device, such as addiction. I will use addiction for the purposes of this theory.
Bilbo is the center of the Middle Earth universe because he is the representation in Middle Earth of the owner of the mind. Bilbos friends take him on the adventure of a lifetime that ultimately provides him with riches and stories to last a lifetime; but along the way Bilbo discovers a vice, an addiction. The ring is the physical representation of the source of addiction and Gollum represents the burden of the addiction which follows the ring.
Bilbo lives years with this perfect device there to comfort him and things are good for him. Then the burden of the addiction, Gollum, sets in motion the guaranteed destruction of the bearer of the addiction. The people that love and care about Bilbo stage an intervention and take the burden unto themselves to rid Bilbo of the addiction and help to stop the utter destruction of his conscious mind that was happening (The Fellowship of the Ring). Once the ring, or addiction, was destroyed, Bilbo left that state of mind.
It is a story of a perfect intervention that leads to a great change in a persons personality developed in a fantastical and mythical representation. Literary masterpiece.