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 Decades later, earnest Catholics were wont to lament it as the regrettable climax of a sad period of outdated and retrograde Mariology (they didn’t know JPII was coming soon).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.