Tom Bombadil is joy and contentment. That’s why:
A) he’s not human. We’re terrible at joy and contentment
B) he’s immune to the Ring. The Ring is temptation, and if you’re happy and content, temptation has no hold on you
C) he was so important to the book plot, which goes further into the metaphor of the Ring than the movies
D) he’s the right thing to cut from the movies, which are more literal than the books
E) he’s so popular in spite of not doing a lot
F) he seems like Eru Iluvatar, being a creature fully made of love and completion
G) he is powerful and yet must be protected
H) he can be summoned through song, yet the song has no power over him. We all have songs that are likely to make us happy, but they don’t always work
I) his enemies are the barrow wights, ghosts that hold forever onto ancient grudges. Those are legit grudges, mind. The Witch-King of Angmar did the men of Arnor dirty. But the wights hold onto those grudges beyond death, and as such are enemies to joy forever. They (the wights) are scattered when their treasures are distributed without history