Why is this person blessed? This Psalm answers this question both negatively and postively.

Not in the counsel of wicked ones: He is blessed because he is characterized by not counseling himself by [wicked ones]. In contrast, since he is not pursuing counsel from wicked people, he seeks counsel from the law of God and those who also are seeking counsel from the law of God.

Not in the way of sinners: This is not a call to abstain from interaction with unsaved people. It is a statement that this man of God is not congregating with willful sinners in the same way he would with believers in true fellowship. He is not trying to gain the very blessings from the unsaved that he would from fellow believers. As Spurgeon says, "he [the righteous person] dares not herd with the multitude that do evil." These sinners are willful in their doing of sin.

Not sitting with scorners: This righteous person is not making himself at home [making his abode] with scorners. For how can righteousness feel at home with unrighteousness?

But rather delights in the law of Yahweh: This righteous person delights in the law of God. How can someone delight in God's law?

Paul says in Romans 7:5, "For while we were living in the flesh, our sinful passions, aroused by the law, were at work in our members to bear fruit for death." Paul also claims in 1 Corinthians 15:56, "The sting of death is sin, and the power of sin is the law." This seems to be a contradiction. Why would the Psalmist say that the characteristics of a righteous man is one who delights in the law of God? Paul answers this question, ⁷ What then shall we say? That the law is sin? By no means! Yet if it had not been for the law, I would not have known sin. I would not have known what it is to covet if the law had not said, “You shall not covet.” ⁸ But sin, seizing an opportunity through the commandment, produced in me all kinds of covetousness. Apart from the law, sin lies dead. ⁹ I was once alive apart from the law, but when the commandment came, sin came alive and I died. ¹⁰ The very commandment that promised life proved to be death to me. ¹¹ For sin, seizing an opportunity through the commandment, deceived me and through it killed me. ¹² So the law is holy, and the commandment is holy and righteous and good (Romans 7:7-12).

We know that the law is good and holy but the law is that which shows us our sin. However, we must realize the law is not the ends for which we should reach. We must also realize the law is not exclusive from the Gospel, while at the same time the Gospel is not exclusive from the law. John Frame expresses this thought this way,

"I do not believe that there are two entirely different messages of God in Scripture, one exclusively of command (“law”) and the other exclusively of promise (“gospel”). In Scripture itself, commands and promises are typically found together. With God’s promises come commands to repent of sin and believe the promise. The commands, typically, are not merely announcements of judgment, but God’s gracious opportunities to repent of sin and believe in him."

The law is that which points out our sin and at the same time points us to The One Who fulfilled it - "For Christ is the end of the law for righteousness to everyone who believes (Romans 10:4, Romans 7:4). The law only has the power to show us our sin; it does not have the power to save. The law is meant to convict us of sin that we may run to Christ for our salvation. With this in mind, the law AND the Gospel must be preached in tandem to our souls. Preaching only law only condemns or produces self-righteousness, and preaching only Gospel makes man think there are no consequences or final judgment to experience. The Gospel is that which enables us to delight in the law of God because the law points us back to our Savior Who is our righteousness.

On his law he meditates day and night: On this law, the law of Christ, is that which this righteous man meditates or more specifically "soliloquizes" by day and night. He is "preaching" the Gospel to himself. He talks with himself regarding the law of Christ both day and night.

He is like a tree planted by streams of water: Jesus claimed, “If anyone thirsts, let him come to me and drink. Whoever believes in me, as the Scripture has said, ‘Out of his heart will flow rivers of living water’” (John 7:37-38). In reality this righteous person "becomes as free being transplanted on rillets of waters" because he is soliloquizing on his Savior.

That yields its fruit in its season, and its leaf does not wither. In all that he does, he prospers: Fruit is a result of what Christ is doing in this righteous person's life. Fruit is not the means; fruit is the end result of what has already been done. "But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, self-control; against such things there is no law. And those who belong to Christ Jesus have crucified the flesh with its passions and desires" (Galatians 5:22-23). This fruit will be produced by the Spirit of Christ; the working of the Spirit is that which makes all he does prosper.

The wicked are not so, but are like chaff that the wind drives away: Not so for the wicked-ones. The wicked ones are as trash being whisked away by the wind. Their deeds will never prosper. This includes their "good deeds" (Isaiah 64:6).

Therefore the wicked will not stand in the judgment, nor sinners in the congregation of the righteous; for the Lord knows the way of the righteous, but the way of the wicked will perish: The wicked ones will not be able to stand in the judgment because they have no righteousness with which to plead before the Judge. They are condemned already in their sin (Romans 1). The Lord knows the way of the righteous because He is the One Who forged the way for all those who trust in Him (Romans 10:9).

To express the first Psalm another way, as Paul states it in straight-forward terms in Ephesians 2, “¹ And you were dead in your trespasses and sins, ² in which you formerly walked according to the course of this world, according to the prince of the power of the air, of the spirit that is now working in the sons of disobedience. ³ Among them we too all formerly lived in the lusts of our flesh, indulging the desires of the flesh and of the mind, and were by nature children of wrath, even as the rest. ⁴ But God, being rich in mercy, because of His great love with which He loved us, ⁵ even when we were dead in our transgressions, made us alive together with Christ (by grace you have been saved), ⁶ and raised us up with Him, and seated us with Him in the heavenly places in Christ Jesus, ⁷ so that in the ages to come He might show the surpassing riches of His grace in kindness toward us in Christ Jesus. ⁸ “For by grace you have been saved through faith; and that not of yourselves, it is the gift of God; ⁹ not as a result of works, so that no one may boast.” Also, Paul proclaims in Romans 5:1, “Therefore, having been justified by faith, we have peace with God through our Lord Jesus Christ, ² through whom also we have obtained our introduction by faith into this grace in which we stand; and we exult in hope of the glory of God.”

Let us exult in the Hope [Christ and His cross] of the Glory of God that we may delight in the law of God which in turn points us back to the Hope of the Glory of God. Oh how blessed is this man "whose hope is in the Lord his God!! (Psalm 146:5).