God is an unchanging, immutable being. From a theological standpoint, this means that God can have no new attributes. The incommunicable attributes stress the absolute being of God, because only He has those attributes. The communicable attributes stress His personal nature by which He enters into relationships with us. These communicable attributes include knowledge and wisdom, along with his goodness, love, grace, and mercy toward us.
These are some of the things that are manifested in the nature of God through His dealings with us. We may organize these communicable attributes into a three-fold classification: mental or intellectual attributes, volitional or will-based attributes, and moral attributes. Under the mental attributes category, we have the knowledge and wisdom of God; in the volitional attributes, we have the power and the will of God; under the moral attributes, we have the goodness, holiness, and justice of God.
So, why can God not change? When we think of change (e.g. from immaturity to maturity), we know that God does not vary in this way, because he always is. We do mature, although there seems to be no motive for this change to occur. Therefore, God need not change, because He already has everything that is necessary for His own being and does not need to mature into something else. He is completely self-sufficient. Consider the writing in the books of Psalms and Malachi.
But thou art the same, and thy years shall have no end. (Psalm 102:27)
For I am the LORD, I change not; therefore ye sons of Jacob are not consumed. (Malachi 3:6)
Has God ever learned anything? Does He get surprised? Does He ever wonder about things? If He knows the past, the present, and the future, does this negate human free agency and will? Can He make the sun shine and not shine at the same time? If all men are created equal, then why are their states and circumstances so different? If God wills all things, does He will sin? All these questions are intended to provoke study of the communicable attributes of God.