29 When you sacrifice a thank offering to the Lord, sacrifice it so that you may be accepted. (Leviticus 22:29 CSB)
While we are no longer under the law and no longer have to provide sacrifices, we are to come before God in thanks.
Our offering now is our prayer and they should be as heavily weighted with thanks as with petitions.
We tend to pray most with petitions, thanksgiving is sprinkled in as a good measure. Instead we should be giving thanks for prayers not yet answered more than anything else.
You see, God always fufills His promises. Maybe not on our timeframe or in the way we would always like, but He is faithful. Thus we should approach our petitions with thanksgiving, showing our faithfulness in knowing that He will answer us.