If the Ascension means the departure of the Lord Jesus, why celebrate it? Who rejoices over the loss of a loved one? Clearly this is not a day to remember what was lost. We celebrate what was gained.
For the first time, our humanity, the nature assumed by Christ, has been taken into the Godhead. This is a coming of age for the human race, something akin to the removal of training wheels.
Here, the sainted scholars of the Church diverge a bit. It’s not clear whether we were created to enjoy the very life of God, or if this is the gladsome result of the Incarnation. Put another way, we don’t know whether the Incarnation, and the resultant glorification of our humanity, happened because of sin, or despite it. Either way, as it did happen, Christ took on our humanity so that we might share his divinity. Today, in him, our humanity is first raised to that height.