Who is Tom Bombadil?
"We are told a startling fact about the Music of the Ainur and water in The Silmarillion, “And it is said by the Eldar that in water there lives yet the echo of the Music of the Ainur more than in any substance else that is in the Earth” and that the elves are drawn to the sea by this echo of the Music. Much in the same way we find Tom absolutely enamored by a Spirit of the water a spirit of the one substance that still holds an echo of the Music. If it is true that Tom is a Spirit of the Music then it makes perfect sense that he is in love with a Spirit of the only substance that still contains an echo of the Music. There is no other place for him to be."
http://whoistombombadil.blogspot.com/2013/01/tom-bombadil-as-music-of-ainur_9.html
"Indeed, Tom must be seen as he has been revealed to us, a rather quirky man who cannot separate anything in his life from song, even his choice of a bride! The best explanation is that he is the Spirit of the Music."