St. Bonaventure bears witness to this, when he says: “The most terrible penalty of the damned is being shut out forever from the blissful and joyous contemplation of the Blessed Trinity.” Again, St. John Chrysostom says: “I know many persons only fear Hell because of its pains, but I assert that the loss of the celestial glory is a source of more bitter pain than all the torments of Hell.”
The evil one himself was made to acknowledge this, as we read in the legends of Blessed Jordan, at one time General of the Dominican Order. For when Jordan asked Satan, in the person of one who was possessed, what was the principal torment of Hell, he answered: “Being excluded from the presence of God.” “Is God then so beautiful to look upon?” Jordan inquired. And on the devil replying that He was indeed most beautiful, he asked further: “How great is His beauty?” “Fool that thou art,” was the rejoinder, “to put such a question to me! Dost thou not know that His beauty is beyond compare?” “Canst thou not suggest any similitude,” Jordan continued, “which may give me to some extent at least an idea of the Divine beauty?” Then Satan said: “Imagine a crystal sphere a thousand times more brilliant than the sun, in which the loveliness of all the colors of the rainbow, the fragrance of every flower, the sweetness of every delicious flavour, the costliness of every precious stone, the kindliness of men and the attractiveness of all the Angels combined; fair and precious as this crystal would be, in comparison with the Divine beauty, it would be unsightly and impure.”
“And pray,” the good monk inquired, “what wouldst thou give to be admitted to the vision of God?” And the devil replied: “If there were a pillar reaching from earth to Heaven, beset with sharp points and nails and hooks, I would gladly consent to be dragged up and down that pillar from now until the Day of Judgment, if I could only be permitted to gaze on the Divine countenance for a few brief moments.”
from: http://www.catholictradition.org/Classics/4last-things3f.htm