The first stanza sets up the poem's main thematic questions: Who created the tiger, how, and why? The speaker in this poem's sister poem, "The Lamb," is able to identify God as the creator of the lamb because the small creature seems to represent joy, love, and freedom—but the tiger is an entirely different figure altogether. The poem does imply that God created the tiger too, but in the tiger's threat of violence and capacity for killing, it's harder for human beings to understand God's motivations for creating it. Essentially, the main aim of the poem is to flesh out this mystery, and to hint at possible answers.
The poem begins with an instance of epizeuxis, with the immediate repetition of "Tyger," which signals to the reader that the tiger is the central figure throughout. And like "The Lamb," "The Tyger" directly addresses the central figure with apostrophe throughout. Indeed, the poem is a kind of awed and fearful meditation on the fact of the tiger's existence. The alliterative "burning bright" creates the visual image of a flash of impressive color moving through the "forests of the night"—which is both a beautiful sight and a terrifying one. Though the poem predates the theories of psychoanalysis put forward by Sigmund Freud and Carl Jung, the "forests" can be interpreted both as the tiger's literal habitat and as a symbol of the human subconscious/unconscious. As this is a poem in part about creativity, the dark and mysterious atmosphere of the forest hints at the mysteries of creation—both in the human and the godly realms.
Lines 3 and 4 introduce the speaker's preoccupation with the creative act. To the speaker, the tiger is too majestic and well-designed a figure to have come into existence by accident. Some "immortal" being must have deliberately created the tiger. Of course, the preoccupation isn't just about tigers. The poem's narrator is really asking why God, as an all-powerful creator with a master plan, decided to create the more fearsome parts of existence as well as the more obviously joyful ones (such as the lamb). Consider the evil that humans inflict on each other, for example—why did God even create that capacity in humankind?
But rather than suggesting God was wrong to create things that seem evil, the poem seems to indicate that elements of God's design for the world are simply beyond the limits of human understanding. People can see evidence of God's divine will, and worship it, but they should never claim to know and understand it fully. Life is full of these mysteries, which is why the rest of the poem consists entirely of rhetorical questions. The poem seeks to illuminate the parts of existence that humans cannot fully comprehend, not in order to explain them away, but rather to marvel cautiously at their presence.