It is not possible that it is the intention of the first two stanzas to nullify the value of man’s efforts to build his house and to protect the security of his city. It would be an offense both to common sense and to the prevailing biblical view that a person must exert effort for the sake of securing his own existence. Rather, man’s own efforts are a necessary but insufficient condition, and in the absence of God’s hidden involvement, man’s efforts will not attain their objective.

The negative formulation utilized to teach this lesson indicates that the psalm is meant to serve as rebuke to people who act improperly and who believe that only through their own strength and might can they build themselves houses and cities, and ensure their own security and livelihood. 

Courtesy of the Virtual Beit Midrash, Yeshivat Har Etzion