"All happy families are alike, and all unhappy families are unhappy in their own way."
The beginning of Tolstoy's Anna Karenina speaks not only to the state of the world's families, but also to the state of the world's literature. The phenomenon of people failing to write compelling happy novels is discussed constantly, and one of the primary reasons for this phenomenon that comes of these discussions is the assertion that uniqueness in stories arises from the story's conflict. Without some conflict to make the characters unahppy, the story will likely fail to be unique and fail to captivate its audience.
This problem has fostered a culture where one can never expect a story to be hunky-dory all the way through. Even a story with a title like Katherine Mansfield's "Bliss" is expected (rightfully) to have major strife.
But in poetry, this is not the case. Plenty of poems successfully capture happiness in a way that is seeminly impossible for longer works of prose. Why is this? I posit this hypothesis, which I implore you to riddle with holes: Whereas prose has to explain a feeling uniquely, poetry has to simply describe it uniquely. In prose, we need to know why the character is happy; a plot event must happen to make the character feel that way, and plot events must keep that character happy in order for this feeling to remain. It is exceedingly difficult to do this and maintain a compelling story. With poetry, however, no justification is necessary; if the poet writes about happiness in an uncommon manner, the reader will be transfixed. As a result, happiness is much more plausible in poetry.

