In ancient Greece, owls were associated with Athena, the goddess of wisdom. She was thought to be accompanied by one at all times, and owls eventually gained their own reputation based on their connection with the goddess a myth that continues to this day.
There is also the common belief that owls simply "look" smart! Unlike most birds, owl eyes are placed on the front of their heads (versus on the sides) to help them focus on prey when hunting. This wide-eyed glance gives them the impression of thinking really hard when, in reality, they are most likely just thinking about their next meal!