Why did our Lord Ascend into Heaven?

Many of us Catholics may have a difficult time articulating the reason why our Lord ascended into Heaven?  Why did He have to ascend in order to accomplish His divine mission of redemption?  What exactly is the effect of His Ascension?  The reason that we fail to understand the answer is one of the chief reasons why we neglect to adequately celebrate the Ascension and Ascensiontide.

Why Did Our Lord Ascend into Heaven?

The answer to this fundamental question is found in the Preface for the Ascension, a Preface found in the Traditional Roman Rite but abandoned in the Novus Ordo.

It is truly meet and just, right and availing unto salvation that we should at all time and in all places give thanks unto Thee, O holy Lord, Father almighty and everlasting God; through Christ or Lord. Who after His resurrection appeared and showed Himself to all His disciples; and while they beheld Him, was lifted up into heaven, so that He might make us partakers of His Godhead. And therefore with angles and archangels, with thrones and dominations, and with all the heavenly hosts, we sing a hymn to Thy glory, saying without ceasing...

The answer to our question is underlined above - "so that He might make us partakers of His Godhead."  Our Lord ascended for us.  He ascended so that we might become divine.

Dom Guéranger, O.S.B. expresses this sublime reason for our Lord's Ascension thusly:

Jesus ascended into heaven. His Divinity had never been absent; but, by Ascension, His Humanity was also enthroned there, and crowned with the brightest diadem of glory. This is another phase of the mystery we are now solemnizing. Besides a triumph, the Ascension gave to the sacred Humanity a place on the very throne of the eternal Word, to whom it was united in unity of Person. From this throne, it is to receive the adoration of men and of angels. At the name of Jesus, Son of Man, and Son of God—of Jesus who is seated at the right hand of the Father almighty—“Every knee shall bend, in heaven, on earth and in hell.”

Dom Gueranger elsewhere reflects as to why the Ascension is always mentioned in the Canon of the Mass. Here he expresses similar noteworthy sentiments:

The feast of the Ascension shows us the work of God in its completion. Hence it is that the Church, in her daily offering of the holy sacrifice, thus addresses the eternal Father: the words occur immediately after the consecration, and contain the motives of her confidence in the divine mercy: ‘Wherefore, O Lord, we Thy servants, as also Thy holy people, calling to mind the blessed Passion of Christ Thy Son our Lord, His Resurrection from the dead, and His admirable Ascension into heaven, offer unto Thy most excellent Majesty a pure, holy, and unspotted Host.’ 

It is not enough for man to hope in the merits of his Redeemer’s Passion, which cleansed him from his sins; it is not enough for him to add to the commemoration of the Passion that of the Resurrection, whereby our Redeemer conquered death; man is not saved, he is not reinstated, except by uniting these two mysteries with a third: the Ascension of the same Jesus who was crucified and rose again.

Heaven was not opened until our Lord's Ascension:

It was fitting that Christ, the conqueror of death, would be the first to open the gates of Heaven. He did so not on the day of His Resurrection but 40 days later when He opened Heaven, led the souls of the just from the Limbo of the Fathers (e.g. Adam, Eve, Moses, Isaiah, et cetera) into Heaven, and He took His seat at the right hand of the Father. For the first time in world history, human flesh had entered Heaven.

As St. Thomas Aquinas stated in the Summa:

"Ascending on high, He led captivity captive," because He took with Him to heaven those who had been held captives by the devil—to heaven, as to a place strange to human nature. captives in deed of a happy taking, since they were acquired by His victory

Reflection from Archbishop Fulton J. Sheen:

"In the Ascension the Savior did not lay aside the garment of flesh with which He had been clothed; for His human nature would be the pattern of the future glory of other human natures, which would become incorporated to Him through a sharing of His life. Intrinsic and deep was the relation between His Incarnation and His Ascension. The Incarnation or the assuming of a human nature made it possible for Him to suffer and redeem. The Ascension exalted into glory that same human nature that was humbled to the death." (Archbishop Fulton Sheen, Life of Christ)