A poem has the potential to open up a special new space, for the reader as well as the writer. What's going on with this stuff? It might be a strange phenomenon.
Themes are so variable that themes may be beside the point. At least as far as that essential strange thing going on. Of course without a theme or inspiration or whatever, a poem might end up a form of non sequitur.
Okay. So what might be happening below the surface?
I'm beginning to think that language itself is happening. Words of their own accord sometimes choose a vessel into which to pour themselves. For the pure fact that language is marvelous and must throw itself an occasional party into lines and stanzas. It must locate a marvelous medium through which to be spoken. Someone like Keats, Emily Dickinson, Pound, Lorca, Pavese, Tomas Tranströmer.
Through some poets, language itself decides to speak. Theme is just a plausible excuse. Metaphors may be swirling all by themselves inside some strange-language-attractor, complexes coming together like genetic material bonding to form variations in potentia. Then they leak out through the fingers of someone sensitively attuned.
The kaleidoscope of imagery, resonance, allusion, rhythm, connotation, and simile is turning and turning, somewhere off in Language Land. Sometimes that scope locks on a target and fires off a wondrous poem.