Strictly speaking, Catholics revere or respect Mary on a par with Orthodox Christians, but somewhat more than most Protestants, who do not believe in invoking saints for intercession (to speak on one's behalf to God). Thus the broad and larger trunks of Christianity consider Mary special, with Protestants the minority exception.
Why? A variety of reasons.
First, there's the respect for the mother of Jesus, which was surely why John the apostle took her to live with him. Second, there's looking at Mary as an example of obedience to God, shown in Luke's narrative of when the birth of Jesus was announced to her. Third, there's the view that thanks to Mary we have Jesus, which has given to various theological titles of Mary, some resulting in a bit of symbolic "inflation," but nonetheless in some respects, reasonable.
People who study popular piety, which is how the theologically untutored devotion to saints and to Mary is called, agree that there are psychosocial factors, as well. Jesus is a powerful male figure. Mary seems more approachable and her story is one of being an ordinary person caught up in a major story, like most of us.
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