We give because God gave first, we want to be more like Him and we trust Him.

"Remember this: Whoever sows sparingly will also reap sparingly, and whoever sows generously will also reap generously. Each of you should give what you have decided in your heart to give, not reluctantly or under compulsion, for God loves a cheerful giver. And God is able to bless you abundantly, so that in all things at all times, having all that you need, you will abound in every good work."

2 Corinthians 9:6-8