"Sonnet 33" begins with a reminiscence: the speaker thinks back on beautiful mornings he's known, when the sun shone beautifully on the mountains.
Already, in the first two lines, something metaphorical seems to be going on here. The speaker personifies the remembered lovely morning: its "sovereign eye"—that is, the sun itself, full of regal power—doesn't just shine down on the mountains, but "flatter[s]" them. It's as if the sun is gazing admiringly at the world, like a lover. And since this poem is a sonnet—a form of poetry that often deals with love—the reader might already get the sense that the relationship between the sun and the world here might be about human lovers, too.
The image of the sun in particular makes that reading especially likely: the idea of the beloved as the sun was so common in Shakespeare's time that it was almost a cliché. (Shakespeare even makes fun of this trope himself in "Sonnet 130.")
If this sun is indeed an image of the speaker's beloved, there's not just romance in the air, but a hint of danger. The word "flatter" has some less-than-happy connotations: it can mean merely to compliment or praise, but it most often suggests insincere praise. In other words: the landscape the speaker is imagining here is already charged with both romance and deceit. This poem will certainly be about love. But it will deal not just with love's glory, but with dishonesty, disappointment, and disillusionment.
There's danger here, but there's also plenty of beauty. The delicate, harmonious assonance of "glorious morning," for instance, makes those mornings a pleasure to hear about, not just to envision. Here at the start of the poem, the speaker is going to luxuriate in memories of what it was like when the sunshine of his life—his beloved—was shining on him full force.