In this case it is the latter part of Verse 3: It is He that has made us AND NOT WE OURSELVES.
Self-made people did not make themselves. Their parents did not make them either. The earth did not make itself. We need to know that. And neither can we sustain ourselves. God made and sustains the world. His world upholds it. All the wild animals are taken care of. All the seeds of plants (and humans) endure and work by his word. We humans need to see that. Why don't we want to be the sheep of the creator of the whole world? A) Because we trust Him to take care of us. B) Because we're too proud to be taken care of.
People always say "The Lord helps those who helps themselves." They say this without qualifier. But it's a phrase that needs qualifying. Even those who "help themselves" are guided by the wisdom of God, are given strength to live by God. God doesn't want us to be lazy, but neither does he want us to be self-reliant. God is good. It's strange to our human mind to think that we have a God who wants to take care of us. But that is the way He wants it. We're an individualistic country and like to think God wants us to take care of ourselves. Humans are judgmental and we want to believe that if someone has a bad lot in life then that person didn't "help himself." And when people have terrible things happen to them, if the person is good we say that God in his sovereignty brought the evil in their lives to teach them. But if the person is someone we consider bad, we say "the wages of sin." That's the trouble with platitudes and maxims. They're human wisdom...and they aren't an answer in all the situations we humans get into.
In all things, we must come to God. For He is good and his mercy endures forever. Or as we say in church. "God is good. All the time."
King James Version
1Make a joyful noise unto the LORD, all ye lands.
2Serve the LORD with gladness: come before his presence with singing.
3Know ye that the LORD he is God: it is he that hath made us, and not we ourselves; we are his people, and the sheep of his pasture.
4Enter into his gates with thanksgiving, and into his courts with praise: be thankful unto him, and bless his name.
5For the LORD is good; his mercy is everlasting; and his truth endureth to all generations.
Psalm 100 (New International Version)
A psalm. For giving thanks.
1 Shout for joy to the LORD, all the earth.
2 Worship the LORD with gladness;
come before him with joyful songs.
3 Know that the LORD is God.
It is he who made us, and we are his [a] ;
we are his people, the sheep of his pasture.
4 Enter his gates with thanksgiving
and his courts with praise;
give thanks to him and praise his name.
5 For the LORD is good and his love endures forever;
his faithfulness continues through all generations.