Why was Margot unhappy on Venus in "All Summer in a Day"?

Margot is unhappy on Venus because she came from Earth and misses the sun. 

The story takes place on Venus, a planet where it rains almost all of the time.  In fact, the sun has not come out in seven years.  Margot, however, came from Earth five years before the story starts. That means that unlike the other children in her class, she remembers what the sun looks like.  She misses it terribly. 

[She]...

Margot is unhappy on Venus because she came from Earth and misses the sun. 

The story takes place on Venus, a planet where it rains almost all of the time.  In fact, the sun has not come out in seven years.  Margot, however, came from Earth five years before the story starts. That means that unlike the other children in her class, she remembers what the sun looks like.  She misses it terribly. 

[She] sensed it, she was different and they knew her difference and kept away. There was talk that her father and mother were taking her back to Earth next year; it seemed vital to her that they do so, though it would mean the loss of thousands of dollars to her family. 

Margot does not get along with the other children.  She doesn’t play their games, and they are jealous of her for having recently been to Earth and for having the chance to go back.  For this reason, the children bully Margot and she isolates herself.  She doesn’t seem to make any friends. 

On the day the sun is supposed to finally come out, the children decide to play a cruel trick on Margot.  They tell her the scientists were wrong, and then lock her in a closet so that when it does come out, she won’t see it.  She is horrified. 

They surged about her, caught her up and bore her, protesting, and then pleading, and then crying, back into a tunnel, a room, a closet, where they slammed and locked the door. They stood looking at the door and saw it tremble from her beating and throwing herself against it. 

Although the children are cruel bullies, the trick they played was terrible.  Margot is very sensitive and this will likely have a great effect on her.  They know this, and seem just as horrified when they realize what they have done.  After the sun leaves, they go to take her out again knowing that she will never be the same.