The psalmist's advice in this verse is an oft-repeated notion throughout Scripture. Psalm 118:8-9 reads, "It is better to trust in the LORD than to put confidence in man. It is better to trust in the LORD than to put confidence in princes." Jeremiah 17:5 puts it even more bluntly, "Thus says the LORD: 'Cursed is the man who trusts in man and makes flesh his strength, whose heart departs from the LORD.'"
This is the essential understanding of this verse: Human beings, compared to God, are fundamentally untrustworthy. While people must be trusted from time to time in everyday life, in the most important matters, however, we cannot afford to lean on the broken crutch of human aid. Ultimately, we are bound to be disappointed because people—even the most well-intentioned—will fail us.
There are several reasons for this. First, people are weak; even the most powerful of men are limited in what they can do. Unlike God, they do not have sovereign control over people, nations, nature, or time. Their limitations make them inconsistent, unable to help when it is needed most.
Second, men are mortal. Several of the other scriptures that warn us not to put our trust other human beings mention that people are here today and gone tomorrow. For instance, Psalm 62:9 tells us, "Surely men of low degree are a vapor, men of high degree are a lie; if they are weighed on the scales, they are altogether lighter than vapor." And of course, the verse after our subject verse reads, "His spirit departs, he returns to his earth; in that very day his plans perish" (Psalm 146:4). Because men live and die so quickly, they lack both the wisdom and the perspective to be trusted on the "big questions" of life. Only God has the eternal knowledge and experience to give us right help and answers we need.
Finally, human beings are unreliable. They blow hot and cold, as it were. They have self-interests that sometimes align with our own and at other times do not. Princes—leaders—especially, do not have our best interests in mind, as they have, not only personal desires, but also political goals to pursue. God, however, though the greatest Leader in the universe, always does what is best for us. Moreover, He is always faithful to what He has promised (I Corinthians 1:9), so if we go to Him and ask Him for help that He has pledged to us, He will give it.
This verse gives us good advice. We would do well to heed it.