Why was the Lady of Shalott forbidden to look down on Camelot?

In Alfred, Lord Tennyson's poem "The Lady of Shalott," the "fairy Lady" lives on the island of Shalott and is under a curse. The curse will be activated only if she pauses from her weaving to look out her window toward Camelot, so she weaves steadily night and day. She has set up a mirror behind her loom that reflects the "shadows of the world" as they pass by her window. 

What is the curse, and why has it been placed on her? The poem is quite mysterious about that. The way she knows about the curse is that "she has heard a whisper say" it. Since she has been called a "fairy Lady" previous to this explanation of the curse, readers know that magic is part of the world she lives in. Interestingly, the Lady knows the action that will activate the curse, but she does not know what the exact consequences will be, as explained by the line, "She knows not what the curse may be." Later in the poem, she leaves her loom to look out her window at "bold Sir Lancelot," and she feels the curse come upon her. She dies as she floats in a boat down to Camelot.

Although the Lady of Shalott was a character in Arthurian legends before this poem by Tennyson, his version of her tale differs significantly from those legends. Elaine of Astolat was a woman who fell in love with Sir Lancelot when he was in disguise and nursed him back to health after he was wounded in a jousting event. Lancelot would not return her love because he was in love with Guinevere. Elaine dies from unrequited love. Tennyson actually used an Italian version of the legend as his source, but he added most of the elements that make the poem so intriguing, including the web, mirror, island, and song. 

It seems that Tennyson was using a shorthand way of expressing the curse of unrequited love. Rather than having the Lady and Lancelot meet, he has a mysterious "curse" take the place of an ill-fated one-sided love affair. Although the chronology is altered, the end result of the Lady pining away and dying for a man she cannot have is the same. Taking Lancelot out of play allows readers to focus on the Lady's plight and emotions completely. 

Although the poem does not explicitly say so, based on the source legend of the Lady of Shalott, readers can interpret the curse as Fate that subjects her to love someone she cannot have; in other words, the curse is unrequited love. However, because Tennyson has left the curse ambiguous, readers can imagine any other source or reason for the curse that suits their fancy.