Because it is the chief part of thankfulness which God requires of us:[a] and also, because God will give his grace and Holy Spirit to those only, who with sincere desires continually ask them of him, and are thankful for them.[b]
a:
Psalms 50:14
Psalms 50:15
b:
Matthew 7:7
Matthew 7:8
Luke 11:9
Luke 11:10
Luke 11:13
1 Thessalonians 5:17