In life we want to cling to someone we love and depend on, we just can't bear the thought of that person leaving us. When Jesus told his Apostles that he was leaving them, they plunged into gloom. He said to them, "It is for your own good that I go away, because unless I go, the Spirit will not come to you; but if I go I will send him to you." (John 14-24 & 25)
It must have been very hard for the Apostles to see how the Lord"s going away could be for their good, yet no one showed more respect for people than Jesus. He did not dominate them, but served them. He gave all people a chance to grow in faith and shine. Had he always remained with them in his physical presence, they would have never come to age themselves. So Jesus sent them his Holy Spirit. He handed his entire world to them an gave them a mission of spreading the good news of the Gospel. What the Spirit did was bring his love awakened energies in them, that they didn't know were there, so they were able to do things they didn't think they were capable of. After Pentecost their hearts were on fire and there was a wind at their backs!
We too need the Holy Spirit so that we can become fearless witnesses for Christ and the Gospel. On Pentecost Day, the Apostles spoke a new language which was the reversed of "Babel". We too need to speak this language. What is this new language? It is the language of peace rather than war; it is the language of cooperation rather than competition; it is the language of forgiveness rather than vengeance; it is the language of hope rather than despair; it is the language of tolerance rather than bigotry; it is the language of friendship rather than hostility; it is the language of unity rather than division; it is the language of love rather than hate. Through the gift of the coming of the Holy Spirit, people of different languages learned to protest one faith to the praise & glory of God. That is the real miracle of Pentecost, and it is a miracle which praise God, still happens.