Chapter II treats “The Participation of the Lay Christian Faithful in the Eucharistic Celebration,” and covers twelve numbered sections (RS 36-47). Translation quibblers may blanch, but the actual subheading of sections 36-42 is indeed, “Active and Conscious Participation.”
As one might suspect, the theological foundation for this is in the documents of Vatican II:
[36.] The celebration of the Mass, as the action of Christ and of the Church, is the center of the whole Christian life for the universal as well as the particular Church, and also for the individual faithful,[Cf. Sacrosanctum Concilium 41; Lumen Gentium 11; Presbyterorum Ordinis 2,5,6; Christus Dominus 30, Unitatis Redintegratio 15; Eucharisticum mysterium 6: GIRM 16] who are involved “in differing ways according to the diversity of orders, ministries, and active participation.[Cf. Sacrosanctum Concilium 26; GIRM 91] In this way the Christian people, “a chosen race, a royal priesthood, a holy people, a people God has made his own”,[1 Pet 2,9; cf. 1 Pet 2,4-5] manifests its coherent and hierarchical ordering”.[GIRM 91; cf. Sacrosanctum Concilium 41.] “For the common priesthood of the faithful and the ministerial or hierarchical Priesthood, though they differ in essence and not only in degree, are ordered to one another, for both partake, each in its own way, of the one Priesthood of Christ”.[Lumen Gentium 10]
Why is active and conscious participation so important? It would seem that it is the way in which the laity exercise their priestly office. To have others exercise their office for them is damaging to the living out of Christian baptism. It is also an offense in the celebration of the Eucharist.
And why is this so? Because Christianity is lived out in worship and beyond, and the connections, especially in this day and age, can be rather tenuous. Many Catholics emphasize preaching the truth. But of what use is knowledge if believers do not make of their lives a tangible example of the Gospel? There’s no better place to start than in the liturgy.