The Lenten season is marked by two very distinct liturgical omissions. Neither the hymn known as the Gloria (Glory to God in the Highest) nor the Alleluia verse sung before the Gospel is permitted during the entire 40 days of Lent (with a few small exceptions).
Why is that?
First of all, the Gloria is a hymn that celebrates the coming of the Lord using words from the angels at Christ's birth. The Church during Lent returns in spirit to a time when the people of God were in exile, waiting for the Messiah to come and save them. It is a similar season of expectation as is Advent, but instead of awaiting Christ's birth from the womb of Mary, the Christian people await Christ's second "birth" from the womb of the sepulcher.
Secondly, following this same spirit of exile, the Church joins Moses and the Israelites as they wander in the desert for 40 years. It is a time of agony and purification, one where the faithful join together in saying, "How shall we sing the Lord’s song in a foreign land?" (Psalm 137:4) The word “Alleluia” is rooted in a Hebrew expression that means “praise the Lord" and is thus omitted during Lent.
Mourning, not rejoicing
As a result, our focus in Lent is not in rejoicing, but in mourning our sins, looking at those things that prevent us from an authentic relationship with God. Once these are removed through prayer, fasting, and almsgiving, we are able to rejoice again at Easter, for it is not only Christ's resurrection that we celebrate, but our own rebirth in the spirit.
Similar to a woman experiencing labor pangs before birth, so the Christian people "groan inwardly as we wait for adoption as sons, the redemption of our bodies" (Romans 8:23).