Like all teachers on human happiness, Jesus tackles the relationship between possessions, necessities, and human happiness. He counsels people to seek the kingdom of God and its righteousness as a first priority in order to cure the anxiety one might have over the future acquisition of food or clothing.
Jesus neither trivializes these needs, as he says to pray for them later in the sermon, nor does he make them absolute sources of happiness or honor as many did in his era. Instead, he says that seeking God’s kingdom and its righteousness will suffice for happiness as well as for the acquisition of the goods of the body.
How can this be so?
On the psychological level, what Jesus is doing is telling people that if they focus upon what they can definitely change, they will not worry. Why is this? Jesus knew one of the great insights of the human mind. You can generally only focus on one thing at a time. If one is focused upon the purposes of God and gaining righteous character, then anxiety (the constant churning of unhelpful and negative thoughts) will slowly cease to be a persistent reality in the human mind.
The other psychological aspect Jesus exploits here is that he tells people to focus on what they can do. The kingdom of God, in the sense used here, appears to mean “the people of God and his purposes for them.” So for Jesus to say to seek the kingdom of God means for people to be busy about accomplishing the commands of God, particularly, the commands which pertain to the wellbeing of other Christians. The second thing Jesus says to seek first is ‘its righteousness.’ That means that character that befits a citizen of God’s kingdom. In other words, seek to become the kind of person who is disposed and poised to act righteously. One has no control over the weather, the crops, the clothing market, etc. But one does have control over their character. By putting people’s minds on the things which they can accomplish (with God’s help, of course), Jesus is helping people to gain an internal locus of control. To have an internal locus of control is to live with a sense that you choose how you handle the world around you and are not by necessity merely the outcome of the events around you. The research is clear that people with an internal locus-of-control struggle less with anxiety.
Jesus observes that for those who seek first the kingdom of God and its righteousness “all these things” will be added. Here he clearly means the goods of the body (clothing, food, human resources in general). But how will they be added? Are we to believe that Jesus is making a promise of miraculous intervention for all do are good enough? I don’t think so. Jesus was aware of the martyrs, Job, and Proverbs and Ecclesiastes. Instead, I think Jesus is making a general claim about the nature of virtue. If you have a good relationship with God’s people and possess righteousness, you’ll generally get what you need. Jesus knew there were exceptions, the Old Testament speaks of them. He also never taught people to expect routine divine protection from harm, he even taught that often, the righteous may have to die for righteousness’ sake. So it appears that here he speaks of the general results of having virtuous habits. Here are the components of “righteousness” according to the book of the Wisdom of Solomon (a Jewish work which approximates Jesus’ thought world quite nicely):
And if anyone loves righteousness, Her [lady wisdom] labors are virtues; for she teaches temperance and prudence,
justice and courage;
nothing in life is more profitable for men than these. 
For those who love righteousness as taught in the Hebrew Bible, wisdom works in them even the four virtues of pagan morality: justice, courage, temperance, and prudence. And while the righteousness of the kingdom of heaven is colored by Jesus’ specific teachings, there is really no reason to suppose that it was no longer seen as an all-encompassing virtue by Jesus or the gospel authors. And besides, “nothing in life is more profitable for men than these” is quite similar to what Jesus was saying.
If somebody have the righteousness of the kingdom, they essentially are growing in the virtues of justice, courage, temperance, and prudence and they can find ways to acquire their needs, stay out of trouble, avoid
Not least, as I hinted at above, people who seek first the kingdom of God, will find themselves in a community of people who will help them through their trials.
Theologically, it’s important to note that Proverbs teaches that:
Riches profit not in the day of wrath: but righteousness delivereth from death.
And for Jesus, no need of mankind was more desperate than the need to know God. And so the central aspect of human happiness here is that those who enter into the kingdom of God and receive righteousness from Jesus Christ will survive the day of wrath.
To seek the kingdom of God and its righteousness first, in my mind, implies that it should be a first priority and driving motivation for our actions but also a temporal one. In our day we should begin with prayer for God’s kingdom to come, forgiveness from sins, and help to do his will. This is why the Christian practice of beginning the day with Scripture reading, meditation, and prayer is so crucial for Christians today. Without this discipline, we’re so likely to rush off into the day and seek anything but God’s kingdom or his righteousness.
 Kurt Aland et al., Novum Testamentum Graece, 28th Edition. (Stuttgart: Deutsche Bibelgesellschaft, 2012), Mt 6:33, “ζητεῖτε δὲ πρῶτον τὴν ⸂βασιλείαν [τοῦ θεοῦ]* καὶ τὴν δικαιοσύνην⸃ αὐτοῦ, καὶ ταῦτα πάντα προστεθήσεται ὑμῖν.”
 The word in Matthew 5:3-10 translated ‘blessed’ or ‘happy’ carries with the connotation of ‘those who are in this state are honorable.’ Matthew 5:3 could be translated, “How honorable are the poor in spirit, for theirs is the kingdom of heaven.”
 The Revised Standard Version (Oak Harbor, WA: Logos Research Systems, Inc., 1971), Wis 8:7.