* Our Sun is Special: We call our star "the Sun" because it's the star closest to us. It's the primary source of light and heat for Earth, making it incredibly important to our planet and all life on it. It's the "center" of our solar system, and we've known about it for a very long time.
* Perspective: For centuries, people were only aware of the Sun. It was the only star they could see in detail. Other celestial bodies were called "stars," but they were simply points of light in the night sky.
* Distinction: Once we started to understand other stars, we realized they were similar to our Sun in many ways. But "the Sun" stuck as the name for our own star, differentiating it from the countless others we see.
* Specificity: Just like we call our planet "Earth" and not "a planet," we refer to our star as "the Sun" instead of just "a star" to be specific.
So, it's a combination of familiarity, historical context, and the unique importance of our Sun to our planet that makes us refer to it as "the Sun" while calling other stars simply "stars."
