Europeans had very limited knowledge of the world beyond their own continent. They knew that there were other lands out there, but they didn't know what they were like. They were afraid of the unknown, and they were worried about what they might find.
Fear of sea travel
In the 15th century, sea travel was very dangerous. Ships were small and fragile, and they were often at the mercy of the weather. Many ships were lost at sea, and those who survived often faced long and difficult journeys.
Fear of other cultures
Europeans were also afraid of other cultures. They didn't understand the customs and beliefs of other peoples, and they were worried about how they would be received. They were afraid of being attacked, or of being taken prisoner.