When reading a cummings poem, it often seems that we "feel" the meaning of the poem rather than actively comprehending the words. cummings toys with this here in his use of language such as "syntax," "paragraph," and "parenthesis," academic terms which stand in contrast to the "feeling" which, the title states, is "first." The overriding theme of the poem, then, is that it can often be difficult to categorize our feelings—and that sometimes it is better not to try.
When we are overwhelmed with feelings, we do not feel compelled to "pay any attention...to the syntax of things," with the speaker telling his subject that a person preoccupied with doing so "will never wholly kiss you." To be too preoccupied with the "meaning" of life and the specifics of what is happening is to divorce oneself from the "feeling" of living. The visceral parts of the speaker understand what is happening: "my blood approves," while being a "fool" is acceptable when "Spring is in the world." ("Spring" may here mean a rhetorical spring, a sense of blooming and possibility.)
Ultimately, the theme of this poem is contained in the phrase "kisses are a better fate than wisdom." Life and love do not need to be constrained to a "paragraph," especially given that death will soon enough take us all—it is no "parenthesis" or unimportant piece of information to be bracketed away, but rather something that will put an end to feeling. As such, the "kisses" should be enjoyed while they can be.