Short-term and long-term memory can be influenced by both internal and external factors.  The time and place where the memory occurred can have an impact on its formation.  Internal factors affecting memory have to do with the neurons in the brain.  If the brain is not developed, such as a in a baby, or is deteriorating, such as in an older person, then the memory may not fully form.  Conditions in the person, such as someone who is tired or scared, can also have an impact.  Adrenaline can impact memory formation.

External factors involving memory can include sensory input.  Smell is  a particularly strong association in memory formation.  Other factors might include the social conditions (who a person was with), location and even time.