From Article 1 of Question 57 of Part III of the Summa Theologica of St. Thomas Aquinas:
Objection: The Son of God took human flesh for our salvation. But it would have been more beneficial for men if He had tarried always with us upon earth; thus He said to His disciples (Luke 17:22): “The days will come when you shall desire to see one day of the Son of man; and you shall not see it.” Therefore it seems unfitting for Christ to have ascended into heaven.
Reply to Objection: Although Christ’s bodily presence was withdrawn from the faithful by the Ascension, still the presence of His Godhead is ever with the faithful, as He Himself says (Matthew 28:20): “Behold, I am with you all days, even to the consummation of the world.” For, “by ascending into heaven He did not abandon those whom He adopted,” as Pope Leo says. But Christ’s Ascension into heaven, whereby He withdrew His bodily presence from us, was more profitable for us than His bodily presence would have been.
First of all, in order to increase our faith, which is of things unseen…For ‘blessed are they that see not, yet believe.’
Secondly, to uplift our hope: hence He says (John 14:3): “If I shall go, and prepare a place for you, I will come again, and will take you to Myself; that where I am, you also may be.” For by placing in heaven the human nature which He assumed, Christ gave us the hope of going thither; since “wheresoever the body shall be, there shall the eagles also be gathered together,” as is written in Matthew 24:28. Hence it is written likewise (Micah 2:13): “He shall go up that shall open the way before them.”
Thirdly, in order to direct the fervor of our charity to heavenly things. Hence the Apostle says (Colossians 3:1-2): “Seek the things that are above, where Christ is sitting at the right hand of God. Mind the things that are above, not the things that are upon the earth”: for as is said (Matthew 6:21): “Where thy treasure is, there is thy heart also.” And since the Holy Ghost is love drawing us up to heavenly things, therefore our Lord said to His disciples (John 16:7): “It is expedient to you that I go; for if I go not, the Paraclete will not come to you; but if I go, I will send Him to you.”