On the Catholic calendar, Assumption Day observes the day Mary died and rose — body and soul— into heaven. The Catholic Church professes that when Mary’s time on earth came to an end, her body was placed in a tomb but her body didn’t decay on earth. Instead, her son, Jesus Christ, assumed her bodily into heaven.
Only God can rise from the dead of his own divine power, and only God can ascend into heaven of his own divine power. So Jesus himself, being God, rose from the dead, but Lazarus was raised by Jesus. Likewise, Jesus himself ascended into heaven, but he also assumed his mother.
So why would Jesus do such a thing? For many reasons:
Jesus loved his mother. Having affection for your mom is as human as it gets. In his humanity, Jesus had all the emotions that any man or woman would have. If you were Jesus, wouldn’t you want to prevent any decay from touching the body of your mother? Wouldn’t you want her to be with you in heaven?
Mary was in a sin-free state. Mary, being free from original sin (through her Immaculate Conception), was also free from the consequences of sin as well — namely, physical death, the separation of body and soul. That being the case, the Eastern Church uses the term dormition (falling asleep) rather than her death before the Assumption. But because she voluntarily joined with her son’s suffering on the cross, Pope John Paul II said that the logical conclusion is that she also willingly followed him through her own death as well.
To encourage the faithful on earth. The Assumption was meant to give humans hope and consolation that what Jesus did for his mother in reward for her being a faithful disciple throughout her life, He will also do this for humankind at the end of time.
At his Second Coming, Jesus will raise the dead and take the righteous to heaven, and the reprobate will go to hell. So, in other words, Mary’s Assumption was like a preview of coming attractions: She was the first human — but won’t be the last — to be assumed by God into heaven. Someone had to be first, and why not the Mother of Christ?