The ease with which a conditioned fear response is acquired can be affected by

Temperament

This refers to the characteristics and aspects of personality that we are born with. These are the traits that are both innate and enduring during our development. During infancy, the person is anxious and nervous. This tends to be the same as the person grows old. The only difference is that the temperament is usually related to emotionality. This emotions are being calm, anxious and nervous.

Depression is lifelong, though you can get medication to with it. the difference between sadness and depression is that sadness lasts a little bit, depression last a long time and it goes on and off. people usually get depression through experiences they have been through, or genetics.