Why is this the answer?

The answer is sun because this is a beautiful and clever riddle that uses word pictures to describe what we see every single day. Let's look at the first big clue: "Inside a great blue castle lives a shy young maid." What is the great blue castle? Think about what is big and blue that is all around us. The great blue castle is the sky! The sky is a huge, wide space that covers our whole world, and it is a wonderful, clear blue on a sunny day, just like a big, majestic castle. Now, who is the "shy young maid" that lives in this blue castle? The maid is a way for the riddle to talk about something special and important that we only see during the day. That is the sun. It is the big star that gives us all our light and heat. Next, the riddle says, "She blushes in the morning." Think about what the sun and the sky look like when the sun first wakes up. Right at sunrise, before the sun is completely bright yellow and high in the sky, the air fills with gorgeous, soft colors like pink, red, and orange. When you see someone blush, their cheeks turn pink or red. So, the sun "blushes" when it rises because it paints the sky with those lovely, warm colors for a little while. It is like the sun is peeking out from behind the sky and being shy about waking up. This part of the riddle perfectly describes the beautiful colors of a sunrise that we all love to see. Finally, the riddle tells us she "comes not out at night." This means the maid goes away when it gets dark. When the sun sets in the evening, it disappears below the horizon, and the sky becomes dark for the night. The sun is only shining on the other side of the Earth then. The sun only shines on us during the day, so it "comes not out" once the night begins. The whole riddle describes the sun's daily journey: it lives in the big blue sky, makes the sky pink and red when it first appears in the morning, and then hides away once the day is completely done. It is a wonderful and fun way to think about the most important star in our sky.