why do you think Lawrence uses words like "dreamily " or "drunken" here?

The words "dreamily" and "drunken," used alliteratively by Lawrence to describe the way the snake looks at him after it "drank" at his trough, serve to emphasize the overall mood of the poem, which drowsily evokes the hot Sicilian afternoon. The alliteration in these words continues throughout the stanza: the snake is "thrice adream" as he begins to "draw" his length away. The "dr" sound itself is drowsy, dreamy; Lawrence also uses alliteration on "s" in this poem—"slowly," "softly," "silently," "slack"—to create a soft, dream-like atmosphere.
These words also personify the snake: throughout the poem, Lawrence does this, describing the snake as "him" rather than "it" and suggesting that he has more right to drink at the trough than the speaker, "like a second-comer," does. The snake is at first drinking "as cattle do," but as the speaker continues to watch, the snake becomes "like a guest" and then "like a god." The "drunken" dreamy aspect of the snake, then, is partly reflected in the speaker himself, who seems half-hypnotized by the snake, "honoured" by his presence.