How come Mary has so many feast days and different names?

Feast days (and solemnities and memorials) are days in the liturgical calendar where the Church highlights and honors an aspect of the Lord, of Mary, or of a particular saint. The Marian days usually fall into different categories. Some of them commemorate a particular event in Scripture, such as the Annunciation or the Visitation. Others highlight various apparitions, such as the feast of Our Lady of Guadalupe or Our Lady of Fatima. Still others relate to dogmatic statements about Mary, such as the Solemnity of the Assumption, or the Solemnity of Mary, Mother of God. Other feast days are based on popular Marian devotions (Our Lady of the Rosary, etc.).
Similarly, the many different titles of Mary acknowledge the breadth of her experiences and influence. Some of her titles relate to her life as the mother of Jesus (Theotokos, or “God-bearer,” or Our Lady of Sorrows). Other titles are geographic, commemorating her apparitions in certain places (Our Lady of Lourdes, etc.) Some of her names relate to the Catholic belief that she continues to pray for us and for our needs (Advocate, Comforter of the Afflicted). There are also many names that center on imagery and symbolism (one very beautiful name is Star of the Sea, indicating that Mary can help us orient ourselves towards Christ).