Because the Church, an authority we can trust wholeheartedly, tells us we ought to
The definition of the Assumption in 1950 caused some dismay. As I recall, Protestants were angry because it wasn’t in Scripture. The Archbishop of York, standing beneath his cathedral’s 600 year old Assumption roof-boss, deplored it as an innovation. The position of the Orthodox was more nuanced: they believed it, of course, but were furious that the Pope had defined it.
Decades later, earnest Catholics were wont to lament it as the regrettable climax of a sad period of outdated and retrograde Mariology (they didn’t know JPII was coming soon).
So what do we believe? “The Immaculate Mother of God, having completed the course of her earthly life, was assumed body and soul to heavenly glory.”
Why do we believe it? Briefly, because the Church tells us, and what the Church teaches from revelation. God teaches. (“O my God I believe in you and all your Church teaches, because you have said it and your word is true”.) What the Church teaches, notice; not the best guess of every theologian. If you have a New Testament handy, look up Mt 16:17-19 and Mt 28:18-20.
But what about Scripture? Briefly, the Church thinks that Scripture, rightly understood, teaches that Mary was immaculately conceived, and that the Assumption would follow. That totally sinless body would not be allowed by her Son to be undeservedly disfigured by decay or any touch of Satan’s work.
Notice in passing that the only reason we believe Scripture tells the truth is because the Church says so. It is the Church which is the “pillar and foundation upon which the truth rests” (1 Tim 3 15); it is the Church which wrote the New Testament, selected the contents, edited it and tells us how to handle it.
Problems from science? We say a “body” (matter) is “in” (a place) “heaven”. Are we not involved in insoluble mysteries? Well, yes, we are. These are the same puzzles we have about the Resurrection – and still more about the Blessed Sacrament. But the puzzles are because we do not understand matter, not because we believe in fairy stories.
Finally, what does the Assumption mean? It means joy, beauty, reward, bounty, the masterpiece of creation… For Jesus, it means that His human love is able to be given totally; for Our Lady, that she can humanly and totally respond to Him. (In heaven there is adoration for our Lord not just by spirits – angels and the saints – but by a real human being with a body). Mary has the reward so richly deserved by her total love. We’re glad for Jesus’s sake, for Mary’s, for the angels and saints who rejoice in their good. And we’re glad for our sake, too: what she has we will have one day – she is our Mother.
More: God salutes a human body. Our own treatment of the human body – Dachau, Dresden, gulags, Hiroshima, abortion, pornography, sex abuse – is not the end of the story.