Haiku Subjectivity and Objectivity
The duality of subjectivity vs. objectivity is a product of Western philosophy, and not native to Japanese thought—or other Asian thought in general, really, in the Hindu, Buddhist, or Taoist traditions. It's mentioned in the Upanishads and the Bhagavad Gita, but mostly to point out that's in error. One could argue that the whole point of the haiku aesthetic is to create unifications, rather than dichotomies: to communicate those moments of transcendence, or epiphany, when the subjective and objectivce unite. So there is in fact a complex layering of consciousness and awareness in haiku, but not necessarily an emphasis on subjectivity.
At the same time, traditionally the haiku aesthetic is for restrained emotion rather than bathos, subtlety rather than blatant emotionalism and drama. It is refined emotion, not divorced at all from the being of the poet and the reader, but evocative rather than openly stated. The angle of a tree, in context, can make one feel longing or heartbreak. Placement in the universe (this Floating World of Dreams, the Ten Thousand Things) in context.
The key is to be evocative rather than to blatantly state things outright. The classic Iowa Workshop piece of advice to writers is "show, don't tell," and as much as that can become mannerist in other genres of poetry, in haiku it's a good tendency to develop. The reader completes the poem, in part, by having a shared group of associations with the poet—not in an artificial way but because poet and reader share cultural and social concepts.
So, we get in haiku the use of season-marker words (kigo), which are essentially plants and animals and insects associated with the seasons, which are prevalent during time of the year, absent at others. The reader who knows that cicadas appear mostly in late summer, knows what time of year in which the poem is set, if it has a cicada in it. The cherry blossoms come in early spring, the leavew turn colors in autumn. Each season of the year has accrued interpretative meaning and associated emotions—autumn is a time of longing and often regret; spring is a time of new beginnings. It's not really all that complicated: it basically consists of the haiku poet giving the reader associative clues that help the reader apprehend the emotional "haiku moment."
In the hands of experienced and good haiku poets, all this is seamless and subtle. At its best, these elements of haiku, which I suppose one could call aesthetic structures, as opposed to strictly poetic structures, all hover below the surface, and arise naturally. In less experienced haiku poets' hands, the scaffolding shows, and you get the sense of "oh, I'm supposed to feel this way because the poet has put in this season-word." It can even seem openly manipulative of the reader's emotions. It's a very subtle thing, and understatement in haiku is usually more effective than overstatement.
The other aspect of worldview relevant to the discussion is the Eastern idea of "beginner's mind," which in my opinion is integral to haiku. This is a state of being, rather than a purely psychological state ("mind" is a loose translation, and does not mean the same thing in Eastern philosophy as it does in Western psychology). It refers to the openness of little children to the world, how they are experiencing everything for the first time, with their full beings engaged. A beginner, when first learning a new skill or way, is dedicated and gives full attention to the task at hand: nothing is assumed, everything is possible, and the best ways of doing a task are yet to be discovered. Everything is done for the first time. Everything is being learned for the first time. An expert, by contrast, knows all the answers already, and so doesn't actually see what's there, but only the overlay of knowledge that is in the mind, projected onto the world. Beginner's mind is open, expert's mind is closed. In the same way, young children don't know all the answers and will often say "I don't know."
Beginner's mind is completely objective and observant while simultaneously engaged emotionally and subjective with regard to the experience of the person. The heart is open to being broken by the tragic beauty of a single ephemeral flower petal falling to the ground. The mind rests serene as the still surface of a pool of water, till the shock of a frog jumping into the pond startles us awake, oh my! It's a state of being in which any event is perceived directly, without judgment of filtering, and in which action happens in the moment, from the center of being, spontaneously and without stopping to think about it first. It has been described as "mind like the wind." It has also been called "mu," nothingness—but it's that nothingness that is filled with the pregnant possibility of being, that moment of raw potential just before the universe came into being.
Beginner's mind tends also to be playful, while expert's mind tends to be serious. You can tell a haiku written from expert's mind, because it tends to be mannered, intellectual, and take itself very seriously. Basho, in talking to his haiku students, emphasized a light touch and a light tone in haiku. Beginner's mind is something to be cultivated in haiku, and experienced haiku poets can still develop beginner's mind even after years of writing haiku. In this sense, haiku-writing is a Way, a Tao or Do, in which the observation of the world one does as a haiku poet serves as practice for getting into and sustaining that state of being known as beginner's mind.
Haiku at its best is a reflection of this state in the writer, and evokes this state in the reader. You experience the poem directly, freshly, without intermediation or interpretation: it just is. This isn't a state that is often described in historical Western literary criticism about poetry; even many Western poets don't mention it; although some writers like Thoreau, and some of the mystics like Meister Eckhart, do talk about it.