Written in the sonnet form, the poem’s main theme is to bring home how important love is in one’s life. Interestingly, Edna St. Vincent Millay deals with this ever popular theme in quite an unconventional manner. She opens the sonnet with the ironic statement - “Love is not all.”

Until line six, Millay defines love in terms of what it is not or what it can’t do or achieve. This is a special literary device named litotes, that employs an ironical understatement in negative in order to further affirm the positive side of something. 

The poet says love can’t fulfill the needs essential to sustain life. For instance, love can’t satisfy appetite for food or quench thirst for water; nor can it cure diseases or save the life of a drowning man. Nevertheless, it’s vital. She says,

Yet many a man is making friends with death
Even as I speak, for lack of love alone

There are a lot of people that embrace death because they haven’t found love in their lives. They may posses every fundamental thing they need to survive, yet "for lack of love" they choose death over life.

The clause “Even as I speak” emphasizes the point that at every moment somebody is choosing death because they haven’t found love. Millay wants to make sure when a reader goes through this line, he or she must realize that at that very moment too, somebody is exchanging death for life as they have failed to find the warmth of love.

In this way the poet wants to emphasize that though love may not meet the basic requirements, it’s still indispensable part of life. Thus, using the clause "Even as I speak," Millay underscores the essential character of love, without which life may be unimaginable.