David was content. His "heart was not exalted." He was humble and resigned to the will of God. (Interestingly, this Psalm chronologically follows "when David was old and full of days" and "made Solomon his son king over Israel" 1 Chronicles 23:1). We act rashly when we exalt our hearts. We presume and act with reckless abandon. We must learn to keep our heart in check, as David did. David knew that "the haughty eyes thou dost bring down," Psalm 18:27. He kept all his senses and heart in check in humility.
David literally says that he did not go among the great ones. He was a prophet, a king. His very life was full of great things. He was even great himself. But this goes with the rest of the verse: nor in wonders above me. He was content to serve God and the Church, and not search into other matters or go higher than he had been given to go. He stayed "in bounds" to his lot. He did not take liberties with his position.
Too many of us busybody ourselves into other things. Things that are none of our business. We concern ourselves with how others do their jobs, whether at work, at church or at home. But it is none of our business. Some of us take on more than we have been given. We take the high seats and step in to make decisions that are not ours to make. Or we puff ourselves up. But it is none of our business. Know your place. Know your duty. Certainly, be diligent and excel within it. But let God give the increase, so you can enjoy His peace and assurance in what you do.
As Paul said in Romans 12:3, "For I say, through the grace given to me, to everyone who is among you, not to think of himself more highly than he ought to think, but to think soberly, as God has dealt to each one a measure of faith."
David compares himself to one being weaned upon his mother. As Jesus said, "Assuredly, I say to you, unless you are converted and become as little children, you will by no means enter the kingdom of heaven" (Matthew 18:3). This infant is not consumed with ambition, but satisfied with simplicity. The world runs with wild passions and finds only discontent. David trusts in the Lord and the Lord compensates him with peace.
When we humbly and soberly see ourselves, when we take our passions and direction from God, He gives us peace.