God is Love. And Love must love. And to love there must be a Beloved. But since God is Existence infinite and eternal there is no one for Him to love but Himself. And in order to love Himself He must imagine Himself as the Beloved whom He as the Lover imagines He loves.
Beloved and Lover implies separation. And separation creates longing; and longing causes search. And the wider and the more intense the search the greater the separation and the more terrible the longing.
When longing is most intense separation is complete, and the purpose of separation, which was that Love might experience itself as Lover and Beloved, is fulfilled; and union follows. And when union is attained, the lover knows that he himself was all along the Beloved whom he loved and desired union with; and that all the impossible situations that he overcame were obstacles which he himself had placed in the path to himself.
To attain union is so impossibly difficult because it is impossible to become what you already are! Union is nothing other than knowledge of oneself as the Only One.