The first section of Psalm 119 brings out something that we aspire to, something that we desire as humanity – we long to be happy, we desire to be fulfilled, we long for blessings. Psalm 119:1-3 gives conditions for one to qualify for these blessings – these are blessings reserved for those who are blameless, those who conform to the will of God, who follow His ways.
In Ps 119:4, we see that God has commanded that His precepts be diligently kept. God has made a demand on us, He has a right to make this demand, as He made us and we owe our existence to Him. God wants us to obey Him diligently and perfectly, according to what He has disclosed to us.
In Ps 119:5, we see that the Psalmist desires the blessings, and desires to live according to the way that God has instructed. He also desires that he may be in conformance with God's command, that he may obey the precepts of the Lord diligently. The Psalmist finds himself deficient. He desires, but is unable to follow through the standard of The Lord. He therefore makes the demand that God has made into a prayer for himself – he longs that God will help him do the very thing that God requires of Him. This is the way of Christians – we have been saved, we have been changed, our hearts have been drawn to God, yet we still live in these bodies of sin (cf. Romans 7). We ought not to despair of this condition, but to long and present ourselves to God that He himself may take us out of the trouble that we are in – the failure to obey Him. However, the obligation does not go away just because we fail to obey.
We are unable to obey God, to do what He requires of us unless He intervenes. This is why understanding of the Scriptures is impossible without The Holy Spirit enabling us to do so. We do not have the ability to consistently obey Him, we do not even have the ability to meet His standard. Without His grace, we can do nothing (cf. John 15:5). The Lord wants us to depend on Him, as anything done without faith is sin (Rom 14:23).
The Lord blesses His people when they long for Him and they desire Him, and they want to do what is right. What God demands of us – which is far above our ability – we can take to Him and ask of Him to help us to live in it. Why then should we go on and live lives that are rebellious to God and full of failure, being tripped by sin? The obligation is there, but the grace is also there. It requires of us to walk in His ways, commit ourselves to Him. The thing that is difficult to do, we can go to God for help, we must depend on Him in every action, in all our ways. Even in prayer (cf. Rom 8:26). We must also seek His counsel with a desire to obey, to put into practice, not just longing, but going on to do what he requires of us.
The desires of an unbeliever to do good are not on the terms that God has given. An unbeliever wants God's blessings without God (cf. Numbers 23, John 6). If you are not a Christian, your desires are not of God, but there is hope for you – God still receives sinners, and is calling you. He requires of you to leave your sins, and if you think that you cannot turn from your sins, go to Him and ask Him to change you, and He will change you.