I've tried to write formal verse. I did a blank verse poem for a class a couple months ago.
I struggle with it. It comes off sounding singsong to me. (My formal verse, that is; it works perfectly well for folks like Philip Larkin.)
When I feel like I'm writing "in the groove," my words sing to me. I feel a sort of musical rhythm about them--and a music to their sound--that probably would be difficult to map out.
It's something I fear losing when I revise poems, especially poems that come to me quickly and mysteriously. And it's something I wish I could insert into poems that sound more labored and prosy to me when I revise them.