I wonder why Roman Catholics show devotion to the Sacred Heart of Jesus and Immaculate Heart of Mary if their devotion can be shown to Jesus Christ and Mary instead? What's the difference?
The Sacred Heart of Jesus and the Immaculate Heart of Mary have been intimately related to one another ever since the moment of Incarnation. Most parents know, especially Mothers, that if there is one thing that connects them to their child, it is the heart. Faithful Christians throughout the Church's history have deductively assimilated the connective reality between the hearts of Jesus and Mary through study of the Gospels, private revelation, and reason. Like all orthodox Christian practices, these devotions flow from Christ's divinity.
The Catholic Encyclodepia presents three foundational aspects for the devotion to the Sacred Heart of Jesus: the historical, the theological, and the scientific:
Veneration of the Heart of Mary is analogous to worship of the Sacred Heart of Jesus. It is, however, necessary to indicate a few differences in this analogy, the better to explain the character of Roman Catholic devotion to the Heart of Mary. The Devotion to the Heart of Jesus is especially directed to the "Divine Heart" as overflowing with love for humanity. In the devotion to the Heart of Mary, on the other hand, the attraction is the love of her Heart for Jesus and for God. A second difference is the nature of the devotion itself. In devotion to the Sacred Heart of Jesus, the Roman Catholic venerates in a sense of love responding to love. In devotion to the Heart of Mary, study and imitation hold as important a place as love. Love is more the result than the object of the devotion, the object being rather to love God and Jesus better by uniting one's self to Mary for this purpose and by imitating her virtues
The Catholic enceycopedia also highlights the slight difference in the historical and theological growth of these closely related devotions:
Ok. I don't know if this will be an appropriate answer, because I have not had the time to dig deeply into finding any expert Catholic writings on this specifically. Pope Pius XII's Encyclical HAURIETIS AQUAS on devotion to the Sacred Heart is about the closest I can find in the near term, but I have not read it fully to see if it addresses your question sufficiently.
By my own reason though, I think of the analogy to understanding a very complex subject, design, etc. in our earthly life or to understanding any given person. If you contemplate a very complex thing as a whole, as humans, it is hard to understand it at a very deep level. If we narrow our focus of the subject in our contemplation, then we start to develop a deeper understanding of the larger subject. I believe that is a large reason that the Catholic Church recognizes various titles, roles, and view points on Jesus, Mary, God the Father, the Church, and even ourselves. It is to allow our limited human perception to penetrate the hugely complex (infinitely complex in the case of God) to understand them much more fully.