Psalm 127: The Difference Between “In Vain” and “Tranquility”

The psalm is comprised of two halves, each addressing a different subject and each with a different atmosphere. The first half expresses the idea that man’s efforts alone will not bear fruit if God is not a partner to his efforts. This idea is formulated negatively: in the absence of God’s partnership in man’s actions, all of his endeavors will amount to nothing.

The second half of the psalm (stanzas 4-7), in contrast, has a different subject: the blessing that comes to a man who has “children of his youth." What is the connection between the two halves? Is there one single idea which the psalm as a whole is trying to express?