God’s anger burns against Moses because Moses tried to excuse himself from doing what God was calling him to do in Egypt. Moses claimed to be slow of speech and tongue. As if Moses should have already known, God asks, “Is there not your brother, Aaron?” Thus, Aaron will speak for Moses.
The fact of the matter is, no man can do the work of God. God is bigger, stronger, and much more capable than we are at anything. No man can fulfill his ministry on his own. I know because I have tried—It’s prideful and arrogant to think we can. It is also prideful to excuse ourselves from serving God because we are incapable by ourselves. I don’t think God ever intended that Moses go by himself into Egypt to face Pharaoh. First of all, God would be facing Pharaoh, not Moses. Moses was simply to be a mouth piece. Second, God already knows that Aaron speaks fluently.
So, with anything we do, especially when it comes to fulfilling our ministries (and everyone has a ministry on this earth), we are not alone. We do not fight by ourselves. God is with us and will not fail to accomplish everything He has set forth. Further, we bear one another’s burdens (cf. Galatians 6:2).* In many ways, unless we are members of a local church with brothers and sisters who will cover our inadequacies and restore us to God where we fail, we cannot fulfill the ministries God has for us on this earth. We need each other. Life is a team sport.
*In context, Galatians 6:2 is about bearing the burdens of one another with regard to sin. If anyone sins against God, we are to work to restore such a person in relationship to God. Such is the case with Moses, here. Aaron compliments Moses’s inadequacies. Where we are inadequate, we have our brothers and sisters in the local church co compliment and restore us to the Father.