"What no eye has seen and no ear has heard, what the mind of man cannot visualise; all that God has prepared for those who love Him …"
(1 Corinthians 2: 9)
Why? because the fullness of the EXISTENCE in God cannot be expressed, or imagine.
God created everything so that it exists. To each creature He gave something of Himself, He gave to each being "a piece" of His existence.
In a specific way it was given to the angels, in a different way to man, in other way to animals, or plants.
But every creature has received from God as the greatest gift "a little bit of existence. "For he created all things so that they might exist; the generative forces of the world are wholesome, and there is no destructive poison in them, and the dominion of Hades is not on earth." (Wisdom 1:14). And every creature was called to improve the existence, to connect to the EXISTENCE OF GOD ever deeper and more closely.
Unfortunately, original sin and all our personal sins put us away from this ideal, from the existence of God and in God. Actually in this life on earth, we participate only fractionally in the existence of God and were it not for God's Mercy we will go further and further from God, more and more in the direction of non-existence, in the direction of evil (and, therefore, the existence is good and its negation or privation is evil - Malum privatio bonum est). Evil is a destruction of existence in being (even only partial). So God is not the cause of evil (the depletion or weakening in existence of created beings). He cannot be, because He is the Pure Existence. Rather the rational and free beings (angels and man) destroy in themselves God's existence. And in this way they are "creating" evil, through their free and selfish choices.
God, then, to turn us back from the road to nowhere incarnated Himself and became a creature, so to heal in us the weakening existence (what we destroyed), to restore us to the fullness of His Existence. And this is our ultimate vocation, our final destination. This is what writes St. Paul in his first Letter to the Corinthians, what we cannot see, neither hear, or understand or express in human words. It is the fullness of EXISTENCE we are called to participate in.
This is exactly Heaven; eternal love, eternal happiness, eternal goodness, eternal beauty. Unum et Verbum, et bonum et pulchrum convertutntur. It is a participation in the fullness of EXISTENCE, the existence of God. This is also the reason that Christ can say, "I and the Father are one." The essence of God; the Father, the Son and the Holy Spirit is the same, i.e. EXISTENCE. And God calls us to participate in this UNITY, in God's EXISTENCE (for human unimaginable, unspeakable).