In Luke 6:38, Jesus says,

Give, and it will be given to you. A good measure, pressed down, shaken together and running over, will be poured into your lap. For with the measure you use, it will be measured to you.

We give because we have freely received.
We give to see God do what only He can through us.
We give in order to experience the fulfillment that comes at the hand of accomplishing our God-ordained role in His advancing Kingdom.
And when we give, we do so with hearts of joy and gratitude.