God is generous and so he calls us to be as well. What we do with what God has given us shows the world where our hearts are at and helps proclaim the gospel. We want to glorify God with every area of our lives, and that includes what we do with our finances.
The greatest act of love and generosity was God giving his son, Jesus, for us. Giving our time, talents, and money is simply our natural response to an understanding of the generosity of a good God.