Before we discuss this question, let’s get the housekeeping out of the way.  Today’s Mass readings (Australia) can be downloaded here:

Word format: Year B Ascension

Pdf format: Year B Ascension

So why did Jesus have to leave his disciples after appearing to them for forty days after his resurrection?  From Fr Barron:

The key to understanding both the meaning and significance of this feast is a recovery of the Jewish sense of heaven and earth. In regard to “heaven” and “earth,” most of us are, whether we know it or not, Greek in our thought patterns. By this I meant that we tend to set up—in the manner of the ancient Greek philosophers—a rather sharp dichotomy between the material and the spiritual, between the realm of appearance and the realm of true reality, between the fleeting earth and the permanent heaven. And if we’re spiritually minded, we tend to think of salvation as an escape from this world—this vale of tears—to a disembodied state called “heaven.” The problem is that these convictions have far more to do with Plato than with the Bible.

Biblical cosmology is not fundamentally dualistic. It speaks indeed of “heaven” and “earth,” but it sees these two realms as interacting and interpenetrating fields of force. Heaven, the arena of God and the angels, touches upon and calls out to earth, the arena of humans, animals, plants, and planets. On the Biblical reading, salvation, therefore, is a matter of the meeting of heaven and earth, so that God might reign as thoroughly here below as he does on high. Jesus’ great prayer, which is constantly on the lips of Christians, is distinctively Jewish in inspiration: “Thy kingdom come, thy will be done, on earth as it is in heaven.” Notice please that this is decidedly not a prayer that we might escape from the earth, but rather that earth and heaven might come together. The Lord’s prayer recapitulates and raises to a new level precisely what the prophet Isaiah anticipated: “the knowledge of the Lord will fill the earth, as the water covers the sea.”   Continue reading here.

Or if you prefer video:

Listen to the homily for today here.