Cummings uses language ingeniously to relate his feelings about love and other aspects of life. In "since feeling is first," a love poem, he uses the metaphor of punctuation to describe his feelings. At the very end of the poem, he states, "for life's not a paragraph/and death I think is no parentheses." These metaphors lend themselves to many in-depth interpretations. For life is not a paragraph, that could mean that life is not a short summary of things; a paragraph is too concise and tiny to hold all that life has. A paragraph is limited; it can only relate so much information. Life is not like that--it is entirely, overwhelmingly full. As he states above, even "your eyelids' flutter" has so much beauty in it that he could go on and on about it for quite some time. Add that to all else that life offers, and indeed, a paragraph does not suffice. Paragraphs also follow an order, a specific format, and life is not that predictable.
For death is no parentheses, think of the times that you use parentheses. Usually, you use it to insert random, sometimes irrelevant tid-bits of information that aren't really pertinent to the main idea or point. You can take the paranthetical reference out and the meaning of the sentence still remains. Death is not like that at all--it is a huge, dramatic, meaningful event. It doesn't just slip in and happen to people and not change the lives of those around them. It is more like a bolded declaration with an exlamation point--DEATH!!--instead of a parenthetical aside--(death). Death is not a side-note on life, it is a definite end, something that changes everything and impacts everyone. To refer to it as parentheses is to ignore its impact and importance.
I hope that helped; good luck!