"'T was so ..." How was the poet-lover so certain in The Good Morrow?

I think that there is a level of certain commitment evident in this poem.  I think that the certainty is because the speaker is committed to the idea that there are not two individuals in love, but rather both voices have merged into one.  The speaker reiterates this theme several times over:  "Let us possess one world; each hath one, and is one" and "If our two loves be one, or thou and I/  love so like that none can slacken and none can die."  The level of certainty is present when one fully immerses their identity within another.  The reason behind this might lie in Donne's metaphysical belief that perfection, or the achievement of the highest representation of love is possible.  Donne does believe in the Platonic ideals or, forms, that indicate all of reality is a constant striving for the ultimate and pure nature of things.  This drive to achieve the form presumes that an idyll is real, and perhaps, the commitment of the speaker in his love might be a method of expressing this; his love is the realization of the form of love.  This drive is beyond certain, for it is absolute in its essence.