When he saw the crowds, he went up the mountain, and after he had sat down, his disciples came to him. He began to teach them, saying: "Blessed are the poor in spirit, for theirs is the kingdom of heaven. Blessed are they who mourn, for they will be comforted. Blessed are the meek, for they will inherit the land. Blessed are they who hunger and thirst for righteousness, for they will be satisfied. Blessed are the merciful, for they will be shown mercy. Blessed are the clean of heart, for they will see God. Blessed are the peacemakers, for they will be called children of God. Blessed are they who are persecuted for the sake of righteousness, for theirs is the kingdom of heaven. Blessed are you when they insult you and persecute you and utter every kind of evil against you (falsely) because of me. Rejoice and be glad, for your reward will be great in heaven. Thus they persecuted the prophets who were before you. (Mt 5:1-12)
Jesus was not just a great teacher. He was more than a teacher. He was not telling them to follow few rules that would make them reach the goal that they wanted to reach. It is true that he had taken the place of a teacher by taking his place on the mountain and making the disciples and the people to sit in front of him. He played the role of a teacher but his teachings were totally of a different nature.
Jesus was not pronouncing the timeless truths about life. He was not talking about human behaviour. (Often the mourners go un comforted and the meek do not inherit the earth). He was announcing blessings and giving wonderful news. He was not giving good advice rather he was giving good news. With the arrival of the kingdom that he was ushering in, this was what would happen to the people.
The kingdom that Jesus was proclaiming was going to change everything and bring in a different way of life. The kingdom of God was for the people who were poor in spirit, who were meek, who were peace makers etc. The world on the other hand was working for success and gain and possession. The people in the kingdom were living a style of life which was different. Hence they were blessed and it was the time to celebrate and rejoice.