The reason why fear consistently seems to be the natural state of being in all of Poe's stories is because his writings are inspired by the literary movement of Romanticism, specifically, Gothic literature.
Gothic literature is a genre that parts from Romanticism, and was commonly used during Poe's time (mid 19th century). Romanticism itself is a genre that uses pure reality as its setting: Nothing is "sugar-coated", nature is admired and revered, and the human condition of imperfection is seen as yet another admirable aspect of life.
Similarly, Gothic literature takes that same aspect of human reality, but pushes it into a different dimension: One in which fate takes over and determines the future of the individual. This uncertainty in plot leads to feelings of fear, inevitability, weakness, solitude, coldness, sorrow, and terror. That is the genre of choice in Poe's work and, because it is his preferred style of writing, fear would be the element that moves most of the action.