Why 'Was Assumed' Instead of 'Ascended'?
Did Mary, the mother of Jesus, ascend to heaven?
...We normally say that the Blessed Mother "was assumed" into heaven rather than "ascended." The reason for this is that using "assumed" emphasizes that it is God who brought her to heaven, whereas "ascended" has as its emphasis the one who is going to heaven. The Church wants to make clear that it is God who brought the Blessed Mother bodily to heaven and not something she was capable of by her own power (unlike the Ascension of Jesus).
The Assumption of Mary is not found in Scripture; it's based on Sacred Tradition. It is something that has essentially always and everywhere been believed by Catholics. The early Christians revered those they considered to be saints. Tombs were places of pilgrimage, and many churches were built with relics. Yet no church or tomb ever claimed to be the current resting place of the body of Mary.
Finally, the Immaculate Virgin, preserved free from all guilt of original sin, on the completion of her earthly sojourn, was taken up body and soul into heavenly glory, and exalted by the Lord as Queen of the universe, that she might be the more fully conformed to her Son, the Lord of lords and the conqueror of sin and death (Lumen Gentium 59).