Because that, when they knew God, they glorified Him not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened.
What happens when we are not thankful? When we no longer recognize God as the giver of good gifts and do not praise Him for the bountiful blessings we receive, we are not honoring the Lord. We are worshiping the creation or ourselves instead of the Creator. If I, who know God yet glorify Him not, become proud and think I have received these blessings by my own efforts, then I am become blind to the truth.
It is a good thing to give thanks unto the Lord, and to sing praises unto The name. Ps. 92:1
O give thanks unto the Lord, for He is good: for His mercy endureth forever. Ps. 107:1