Author Ray Bradbury purposely wrote a story about a group of children living on Venus in constant rain showers that is meant to bring about questions in the minds of its readers. The question of why Margot feels differently about the weather on Venus could be answered in several ways, all with textual evidence to support the answer.
For one thing, we find out right away that Margot is an outsider to the group of children introduced in the story from the beginning. Although we do not know why she does not get along with the others right away, we do eventually find out that the other children were born on Venus while Margot only recently arrived there. This alone is enough to make Margot feel differently about the weather. These children were all born nine years ago on Venus and the constant rain is all that they know. Though the constant rainy weather is an annoyance, it is not anything the children seem terribly upset about. Margot however, only came to Venus a few years ago and lived on Earth where the sun still shone until she was five. Margot remembers the sun and clear skies. This makes the rain on Venus almost intolerable. She remembers the sun, it's warmth and beauty so the constant rain is horrible for her.
Additionally, she probably resents the rain. The author wrights that the other children are jealous of her, bully her and make fun of her because she says she has seen the sun. This probably makes her resent the rain on Venus even more.