"Poetry is a spontaneous overflow of powerful feelings." - Wordsworth

I believe this quote best describes poetry. The most powerful time to write is when you're in the midst of feeling strong emotions (anger, sadness, love, pity, jealousy, etc.) Wordsworth describes it perfectly when he describes it as a "spontaneous overflow" - you never know what you're going to write about. It may begin one way, and veer into a completely opposite direction by the end of the poem.