The speaker begins by suggesting that "feeling is first." This means that people experience feelings and emotions before experiencing anything else. In other words, people process life on an emotional level before analyzing their experiences on a logical or rational level.
With this in mind, the speaker wonders who would bother to stop and think about "the syntax of things / will never wholly kiss you." This is a metaphorical question, as the speaker uses the word "syntax"—which basically refers to the arrangement of words in a sentence—to imply that overthinking arbitrary details is a useless thing to do, especially when these details will never bring the kind of happiness or satisfaction that comes from a good kiss. In this way, the first stanza ("since feeling ... wholly kiss you") lays out the speaker's belief that feelings are more valuable and rewarding than thoughts.
The speaker's combined use of alliteration and sibilance in these lines also make them sound pleasing to the ear. For instance, the /f/ sound repeats in the first line:
since feeling is first
This alliteration connects the word "feeling" to the word "first," and this ultimately helps the speaker emphasize the idea that people experience things emotionally before experiencing them in any other way.
What's more, sibilance runs throughout the first stanza, as the speaker uses not only the standard sibilant /s/ sound, but also the /z/, /th/, and /f/ sounds:
since feeling is first
who pays any attention
to the syntax of things
will never wholly kiss you;
This sibilance creates a satisfying hissing sound, one that makes the speaker sound relaxed. This tone is appropriate for the poem, since the speaker's general attitude is one of happiness and contentment. In turn, the sound of the speaker's words reflects the poem's overall mood.
On another note, the first stanza establishes the poem's lack of meter and form. Although these four lines are all roughly the same length (containing between five and seven syllables), they don't adhere to any kind of meter, nor do they set forth a rhyme scheme. As such, the first stanza establishes a free-flowing style that continues throughout the rest of the poem.