I started praying the Rosary after St. John Paul II added the Luminous mysteries to the Rosary, so I virtually no remembrance of a Rosary being prayed before they came on board. When he writes about them in Rosarium Virginus Mariae he kind of just slides them in and says what he thinks they should be.
Moving on from the infancy and the hidden life in Nazareth to the public life of Jesus, our contemplation brings us to those mysteries which may be called in a special way “mysteries of light”. Certainly the whole mystery of Christ is a mystery of light. He is the “light of the world” (Jn 8:12). Yet this truth emerges in a special way during the years of his public life, when he proclaims the Gospel of the Kingdom. In proposing to the Christian community five significant moments – “luminous” mysteries – during this phase of Christ's life, I think that the following can be fittingly singled out: (1) his Baptism in the Jordan, (2) his self-manifestation at the wedding of Cana, (3) his proclamation of the Kingdom of God, with his call to conversion, (4) his Transfiguration, and finally, (5) his institution of the Eucharist, as the sacramental expression of the Paschal Mystery.
So, does anyone know what prompted him to add more mysteries? He explains why he chooses the individual mysteries in the letter, but he doesn't really say why he thinks that the Rosary needed more stuff.