Thanksgiving is what entitles a believer to his next levels of God's blessing. It was guarantees the believers' increase. Let the people praise him, let the people praise him then shall the earth yield her increase Ps 65:5-7.
Giving thanks is one of the ways of showing appreciation for what God has done and what He is about to do in our lives. You can thank God in anticipation of what he is about to do in your life.
Why must we give thanks?
There is nothing that we have today that is not giving him from Heaven. Who supplies our needs from heaven. Our heavenly father. So whether it be children, house, wealth, health, and all the good things of life, it must be given from heaven.
James 1:17 Every good gift and every perfect gift is from above, and cometh down from the Father of lights, with whom is no variableness, neither shadow of turning.
God is the father of lights and we are the light of the world. My emphasis here is that whatever we are and have today is not an achievement but a receivement from the Lord.
Jesus is Lord.