The answer to this lies in part in understanding the historical importance of Midsummer festivals, upon which Shakespeare's A Midsummer Night's Dream is founded. Midsummer festivals were still a large event in Elizabethan England, although the pagan festival had been Christianized with an emphasis on John the Baptist instead of on the Summer Solstice.
Midsummer festivals historically celebrated the gods and emblems of the solstice. Bonfires were set to honor the Sun, at its apex at summer solstice. Maidens historically went to bed that night with folk magic rituals and charms to bring their true loves to them. This had symbolic association with the first harvests, especially so in northern climes with short growing seasons. So, love, magic, dreams, and power (as from the Sun) were overriding themes at play during Midsummer's night. This gives a brief historical perspective as to why dreams play such a large part in Shakespeare's play.
Further understanding stems from a brief textual analysis. Shakespeare opens the comedy with dialogue between Theseus and Hippolyta who establish the dream and love themes. Theseus speaks of their approaching wedding ("our nuptial hour / Draws on apace") that will be held at the time of the full moon ("four days brings in another moon"). He then complains that the current moon changes phases too slowly for his happiness, "she lingers my desires." In these lines, Theseus introduces the idea of love through his upcoming wedding day and of his delayed "desires." He also introduces the idea of dreams through the association with the moon--what else does one do when the moon is on the rise? One sleeps. One dreams.
When Hippolyta responds, she reinforces both ideas about dreams and love. Her first remarks are that the days "will quickly steep themselves in night" and that the "nights will quickly dream away the time." Her closing line gives a feminine expression to the sentiment Theseus referred to as "desires"; she says the moon shall soon "behold the night / Of our solemnities." So through her dialogue, Hippolyta confirms the importance of the themes of dreams and love in the upcoming play.
When you combine the importance of the themes of love and dreams in the play with the understanding of the importance of Midsummer festivals, it is easier to see whether dreams are a negative thing or not, and whether or not this play presents the dreams of the play as a departure from reality. Since dreams are an important part of Midsummer festivities, and since the text of the opening lines of the play establish dreams and love under the Midsummer moon as desired, sought for, and happily anticipated, and since dreams fit precisely with the reality of the historical activities of Midsummer festivals, it may be stated that dreams in this comedy are not negative and are not meant to be seen as a departure from reality--we may have departed from the reality of ancient celebrations of Midsummer festivities but for Elizabethans Midsummer festivities were not seen as a departure from reality.