Why do we feel nastolgia?

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Across
  1. 2. The time that has already happened (4 letters).
  2. 5. A view or attitude toward a situation or event, often influenced by past experiences (9 letters).
  3. 7. The act of thinking deeply about something, often leading to nostalgic feelings (10 letters).
  4. 9. A complex psychological state that involves a subjective experience.
  5. 10. A strong desire for something that is lost or absent (7 letters).
Down
  1. 1. A state of well-being and contentment often associated with positive memories .
  2. 3. The indefinite continued progress of existence and events (4 letters).
  3. 4. The act of becoming different, often leading to feelings of nostalgia .
  4. 6. The qualities, beliefs, and personality that make a person who they are, often shaped by past experiences
  5. 8. The mental capacity to store and recall past experiences (6 letters).