The heart in its journey towards Allah subhanahu wa ta'ala, is like a bird whose head is love, and hope and fear are its two wings.
When the head and the two wings are sound and healthy the flight of the bird is good, but when the head is cut off, it immediately dies, and when either or both wings are deficient, the bird cannot properly fly and may become victim of any hunter or snare.
The righteous predecessors preferred to strengthen the wing of fear during good times when heedlessness is feared, and to strengthen the wing of hope at times of calamity and when near death.
Some have said that it is better to strengthen fear more than hope because when vain hopes overcome a person he is ruined. Others say that the best of situations is a complete balance of hope and fear with overwhelming love, for love is composite, while hope is a sharpener and fear a driver.
Ibn ul Qayyim al Jawziyyah (Rahimahullah) - The Station of Fear