This is the way in which a true encounter with God affects an honest soul. Isaiah is in a state of spiritual shock. He has in vision been transported to the center of God's heavenly temple throneroom. There he encounters a holy God and is overwhelmed with his own unworthiness to stand there. He is utterly lost. There are three reasons for Isaiah's shock...
REASON ONE: His own sinful state. Isaiah is personally aware of his own lack of holiness when he sees a glorious God whose majesty fills a temple with His glory which then spills out to cover the entire planet! Isaiah is overwhelmed with an awareness of his own lack of holiness. He is a man of "unclean lips". As a lost man he is not capable of singing with the seraphim, "Holy! Holy! Holy!" He feels as if he should not utter anything in the presence of such holiness and majesty.
REASON TWO: The sinful state of Israel. Isaiah knows that this kind of God has the right to judge a sinful people. And Isaiah, a sinner himself, dwells among a rebellious and sinful people who have rejected God. He is overwhelmed by this realization. None of his contemporaries could stand with him before God. Only God is truly holy.
REASON THREE: The vision itself. When Isaiah saw God, he nearly had a nervous breakdown. His ability to process the experience was shortcircuited by the fact that he had encountered The Lord of hosts. He could not get beyond that fact. It broke him as it broke into him. He was beside himself, shattered by an awareness of His sinfulness, the sinfulness of his world, and the mighty holy power of a glorious God. When I hear casual stories of visions of God where the human teller of the tale seems to be the center of the experience (AKA the rash of recent "I went to heaven" books), I smell a big fat stinking lie. Period. These people are lying. Encountering God is humbling and breaks you down. Ask Isaiah. It took a holy angel with a burning coal from heaven's altar to touch his lips and get him out of his sinfulness with a temporary atonement to change him enough that he would write about the vision! When we encounter a holy God, we are broken and changed by that knowledge.