First and foremost, God wants us to give because it shows that we recognize he is truly the Lord of our lives.
Every good and perfect gift is from above, coming down from the Father of the heavenly lights, who does not change like shifting shadows. James 1:17, NIV)
Everything we own and everything we have comes from God. So, when we give, we simply offer him a small portion of the abundance he has already given to us.
Giving is an expression of our thankfulness and praise to God. It comes from a heart of worship that recognizes everything we have and give already belongs to the Lord.