The Sun & Leo
Leo
The symbol for Leo is, of course, the Lion. Lions seem to have caught our collective attention early in our history and have been attributed with ferocity, power, and strength as depicted by drawings on cave walls of lions hunting. A book written somewhere between the 2nd and 4th centuries referred to as the Physiologus is sort of an ancient bestiary, listing all kinds of animals along with stories about them from which we can derive the meaning of the animal as a symbol (typically allegories conveying Christian principles). This book has had a tremendous and long lasting effect on the symbolic meaning we ascribe to animals. The Lion has two chapters in this book and conveys the notion of the 'King of Beasts,' as if the Lion is a symbol of Christ himself, a spiritual king.
The typical Greek attribution of the symbolism of the lion is the Nemean lion, which was a fierce animal whose hide could not be penetrated. Heracles fought and killed the lion as the first of his twelve tasks he had to perform as a penance. Not a very romantic story if you're the Lion!
The idea of lion as king, though, is long lasting and has lead to the prolific use of the lion in heraldry (designing coats of arms, in this reference), so the lion has continued to be attributed with the qualities of a 'kingly' nature such as strength, valour, bravery, and someone who is a protector. Even the sphinx is a lioness body with a human head, combining the traits of a human mind with the power and royalty of the lion body. Therefore the traits given to Leo through the association of the lion as king are easy to see: one who commands through the magnetism of his or her presence alone through charisma, generosity, nobility, and pride.
The Sun
Every culture has symbolism, often creation symbolism, around this heavenly body. Mithra is the Persian god of light, Ra, the Egyptian sun god, and Sunna is the Norse sun goddess., to name a few examples. The Greek god Apollo is attributed to be the god of light, and the Roman god Helios to be the sun personified, although the two are essentially interchangeable at this point, blurring into one symbol with two names. Helios was said to have a shining light around him just like the crown of the sun, and drove the chariot of the sun across the sky with four horses pulling it through its daily cycle.
The symbolism surrounding the sun predates the written word. The obvious things we can observe about the sun are the traits we can find still attributed to it in astrology: it is the center of all. The sun gives and sustains life. It is a light giver. The sign in which you find the sun in your chart describes the things you need to do and develop in order to sustain your own vitality and center of gravity, figuratively speaking. The sun is an appropriate symbol for Leo as Leo's presence cannot be ignored and when healthy, lights up the room, as does the sun. Yet, just as if the sun were to get too close and burn up the earth, so does unhealthy Leo burn up those surrounding it with overpowering and prima donna behaviors.