The history of civilization in the West (i.e. Europe) goes back many thousands of years. Since well before 1000 BC there have been people living in what is now Turkey, Greece, and even Italy, in such cities as Miletus and Troy. For much of their history, people living in the West were content with their traditional mythology and legend as explanations of the world around them. In the 7th century BC, Homer came around and developed perhaps the greatest collection of myth and legend in Western literature. However, it was just that: myth, quite unsubstantiated and irrational, though incredibly important nevertheless.
But then something seemed to change. In the late 7th century BC there came a man named Thales, and he was not satisfied with the traditions of old. Instead, he developed his own theories on what the world is made of, declaring among other things that "water is the first principle" from which all things come, and that there is a single entity "God" which "shaped and created all things from water."
And he wasn't the only one determined to come up with a more reasoned and physical explanation of things. Soon there came Anaximenes, Parmenides, Pythagoras, Anaxagoras, all of whom had their own decidedly un-mythological theories of the world. And with that the discipline of Ancient Philosophy was born, and soon there were tens, hundreds, thousands of people who at least tried to work things out on their own.
Why, though? When people had been living with myth for many centuries, why did they start to look for their own answers, independent of tradition or legend? Admittedly some of their theories were still quite absurd, but at least they generally didn't involve various gods and goddesses giving birth to the Earth and its creatures. The new theories were much more physical and separate from the gods, and they tried to break down what the world is made of, how it came to be, and how things should work in a much less divine sort of way (though God or the Mind or some other entity often played a role in cosmogony, the field that is still quite a mystery to us today).
So, what led to the birth of philosophical thought as opposed to mythological tradition in the Ancient West?