First of all, we can clear the air about questions from our Bible-based brothers and sisters: no, the Assumption is not found in the Bible (at least, not as such). But it has been believed in/celebrated (especially in the Eastern Churches) since very early times. How did it come to be?
The vision of Revelation 12 is a beginning. And the words of Jesus to the “good thief” (Luke 23) are another point. We could also consider the parable of the Rich Man and Lazarus (Luke 17)—Jesus was happy to describe the place of Father Abraham and Lazarus in heaven. So besides Jesus, holy people (or forgiven people) are in Paradise with the Blessed Trinity “in refreshment, light, and peace.” Is it out of line to think that the Blessed Mother (“full of grace; the Lord is with you”—Luke 1) would not also be there? Would Jesus deny to His Mother what He promises us all?
This devotion far precedes agreement on any other Marian beliefs, including that of the Immaculate Conception, hotly debated in the Middle Ages (affirmed by Franciscans like Duns Scotus, though rejected by Dominicans—like St Thomas Aquinas).
If it has been believed for well over a millennium and a half, why was it only in 1950 that it was proclaimed infallibly (ex cathedra) by Pope Pius XII? Why was it necessary? I think part of the reason is the historical context—the world was emerging from unspeakable horrors: World War II, the Shoah, the devastation of the atomic bomb, the fire-bombings and saturation bombings of Tokyo and Dresden, among other places, the sheer numbers of the slaughtered (all of which, very likely, could have been prevented save for the punitive vengeance of the Treaty of Versailles which formally ended World War I). People were becoming convinced that this is who we are and that there is no future (in the words of an old song by Peggy Lee: “Is that all there is?...Then let’s keep dancing…”). The Holy Father wanted to affirm that that isn’t all there is—that creation and the body have a destiny of restoration, of transformation, like to the Risen Christ, and that ALL of creation is redeemable and in process of being redeemed (no matter how terrible the present times look). Re-read Romans 8 for this sense.
So: what is conferred on Mary is not, so to speak, a special blessing; it is the blessing promised to all of us. What is unique for her is the timing. We need to wait for the Final Consummation; she, by virtue of her Son and her special role as Mother of God, enjoys our destiny now. But precisely because it is also our destiny, we celebrate her knowing that soon we will join her.
So let’s celebrate the Assumption, knowing that we are also celebrating our own eternal future in Him, the one Redeemer of us all.