Leo is one of the oldest constellations and was seen as a lion ever since, and as old as this is the human tendency to view a lion as something very powerful and royal. In ancient times, the sun entered Leo in the hottest time of the year, thus creating the association to heat, and in old Egypt it was the time of the annual Nile flood, therefore the reputation of honor and generosity. Also, the constellation shows the bright shining star Regulus, sometimes taken as the Eye of Ra in the night.
The ancient Egyptians had a powerful and fierce lion
goddess called Sekmeth which means 'The mighty one', she was seen either as a lion
or a woman with a lion's head, born out of Ra's eye which he threw down on earth
when in rage about human evil, and the fiery eye turned into the furious Sekhmet -
which explains why the 'eye' Regulus is found at Leo's feet.
Sekhmet became the goddess of war who was told to support the Pharaoh in wartimes, an image of fury and glory, called the 'lady of pestilence' for slinging plague, disease and destruction. Beside this, she was honorful and generous and known to be a healer of the ailments she had caused herself, further she was told to be the protector of the bold and righteous - the only problem with Sekhmet seemed to be that she lost her most passionated temper a little too quick.
In Greek mythology the Leo was taken as the Nemean lion, which could not be killed by metal or stone but of course could get killed by the hero Hercules who used the lion's skin and claws to make himself some new weapons.