I will be a father to him, and he shall be a son to me. As a people of God, we are always called into a relationship with God. We come from God, and it is God who holds us in being. Whenever we read the Scriptures, we are always reminded of God wanting to be involved in our lives, as is clear from the covenants God has always made with God's people. The covenants serve to remind men and women of every generation how close to God's heart they are. This is clearly seen in the one phrase that keeps recurring whenever God makes a covenant with the people: " You will be my people and I will be your God" (cf. Gen 12: 2, 17: 8; Exodus 6: 7; Leviticus 26: 12; Jer 30: 22). The only condition God gives men and women as partners to the covenant is that they abide by the terms of the covenant: remaining in a relationship with God. Most of the time we forget the terms of the covenant in our seeking to remain in this relationship. We forget that God has sought us, and that it is God who has invited us into this relationship. We often forget that God only wants our fidelity to the terms of the covenant, nothing more. 
This is what God seeks to remind David through the prophet Nathan in the first reading today. David was seeking to build God a house. He wanted  to do something for God. God had been so good to him, and in thanksgiving, David wanted to build God a house. God appreciated the offer, but said no. All that God had done was to fulfill God's promises to David. All that God was doing was to remain faithful to the terms of the covenant that God had made with David’s ancestors. If God was being good to David, it was not a favor that God was doing. This is what God does. This is who God is. Good is the other name of God.  I will be your God, God had said. 
Many times we get to become like David. We get overwhelmed (and even surprised) at God's show of goodness toward us. God is reminding us today that we should not get overwhelmed. We only need to recall the terms of our covenantal relationship with God and realize the fidelity of God. On our part, God only asks that we also remain faithful to the terms of the covenant: remain in a relationship with God (be God's people).