For me, there isn't a specific "why" (as opposed to the above "how") for writing poetry or pretty much anything else. I am reduced to the metaphor of Donaldson's "wild magic"-- sometimes it just happens, though I'm pretty sure I enjoy it more than Thomas Covenant. (Yes, both parses are valid, though I suspect that using "parse" as a noun is not.) More often, it doesn't, but hey.

At least the polysemy angle tells me why I was so dissatisfied with the terzanelles (http://www.uni.edu/~gotera/CraftOfPoetry/villanelle.html) I tried writing as a moody/bored teen; at the time, I wasn't entirely conscious that the repeated lines were just shuffling around indigestibly piecemeal, but I knew *something* just wasn't right.