Why would anyone constrain themselves to writing poetry in Haiku?
Why would Shakespeare force himself to write entire plays in iambic pentameter? This makes no sense. Think about how much harder it would be to write a play where each line has ten syllables and rhymes with every other line. It’s a daunting challenge in itself. Now write a dozen of them, and make them into some of the best stories ever told.
But, perhaps part of the reason Shakespeare was so good was precisely because he forced himself into these positions:
Unless poets are stumped by the form, unless they are forced to look beyond the obvious associations, they’ll never invent an original line. They’ll be stuck with cliches and conventions with predictable adjectives and boring verbs. This is why poetic forms are so important. When a poet needs to find a rhyming word with exactly three syllables, or an adjective that fits the iambic scheme, he ends up uncovering all sorts of unexpected connections. The difficulty of the task accelerates the insight process.
You break out of the box by stepping into shackles.
~ Jonah Lehrer, Imagine: How Creativity Works
While at first this sounds counter-intuitive, it shouldn’t be that surprising. We’ve all experienced this with deadlines, thinking of something great at the last minute just because we had to. We’ve all experienced it with wordplay, when someone says a particular word or phrase and you associate it or twist it into something completely different.
So, force yourself into some constraints. It’s uncomfortable, and you’ll struggle, but when you finally succeed, you’ll actually have produced something new.