Why did Jesus leave his apostles and disciples for heaven? Did he have to? Why did Christ promise to send the Holy Spirit while he departed from the earth? Could he not stay forever to govern the Church?

I think that he ascended to heaven because it was the best for his people, his Church to receive salvation. Every way the Word of God chose on earth was for the best and easiest way for man. He could have saved man without the Incarnation, the Passion, and the Resurrection. But the Son of God chose these ways because man could access the work of salvation with human senses and mental capacity. He chose the ways a man can see and hear and touch and understand. By becoming a true man, he opened the way a man can participate with his human faculties.

Then, what about the Ascension?

We do not like to say goodbye. We have a hard time losing our beloved ones. However, to become an independent and complete human person, one needs to be alone. The apostles and disciples could not have matured if they could have always depended on the Lord. They would not have become his witnesses if Christ had been on earth forever. But more seriously, they could not have interiorized the faith if Christ had remained as an external subject while the Holy Spirit had not descended and dwelled within them. Ironically, for Christ to dwell in every Christian, he had to depart from them into heaven. Then, the Holy Spirit could come down and accompany every Christian.

Do we stand only looking up toward heaven? Now is time to witness Christ. It is time to grow and bear fruit.