The Athenians were fiercely proud of their independence and their democratic system of government, which was unique in the ancient world. They saw the Persians as a threat to both of these things.
The Persians were a powerful empire that had conquered many other nations. They were also ruled by a single monarch, who had absolute power. The Athenians feared that if the Persians conquered Athens, they would lose their freedom and their democracy.
In addition, the Athenians had a strong religious and cultural identity. They believed that their gods were superior to the gods of other nations, and they saw the Persians as barbarians. They were determined to defend their religion and culture from what they saw as a foreign invasion.