Blessed are those who hunger and thirst for righteousness, for they shall be filled (Matthew 5:6).
There is a thirst and a righteousness that promises complete fulfillment. That hunger and thirst is that which has to do with God. Hungering and thirsting for God is the right kind of need. That is what Jesus wants us to see in Matthew 5:6. Those who are in the kingdom are those for hunger for these things.
There are many people who pursue happiness in the wrong places. Lucifer, whose name means "star of the morning" or more literally, "the bright One," was not satisfied with living in God's glory. So he said in his heart, "I will ascend above the heights of the clouds, I will be like the Most High" (Isa.14:13-14). His ambition was not to reflect God's glory but to usurp God's sovereign power -- while forsaking righteousness. God's response to his ambition was "yet you shall be brought down to Sheol, to the lowest depths of the pit" (v.15). There's another person who had mis-directed ambition. In Daniel 4:29-32 you read about Nebuchadnezzar. In this passage you see that his hunger was for praise. Therefore God humbled him until he knew "that the Most High rules in teh kingdom of men, and gives it to whomever He chooses" (v.25). Even in Luke 12:16-21, in the parable of the rich man, we find someone like Necuchadnezzar and Lucifer who were hungry for the wrong things. The rich man was pleasure hungry. Because they all hungered for wrong things and rejected God's good things, they forfeited both.
"Jesus declares that the deepest desire of every person ought to be the hunger and thirst for righteousness. That is the Spirit-prompted desire that will lead a person to salvation and keep him strong and faithful once he is in the kingdom. It is the only ambition that, when fulfilled, brings enduring happiness" (John MacArthur, The MacArthur New Testament Commentary: Matthew, p.178).
Do you thirst for God's righteousness? Do you hunger in the sense that you have "a passionate longing for something about which one cannot live?" (Colin Brown). As Psalm 119:20 speaks of a soul that "breaks with longing for Your judgments at all times," may your heart be filled with a hunger for His righteousness today as you serve Him.