Richard Solomon's opponent process theory (1974) proposes that emotional experiences trigger an opposing emotional response that stabilizes the nervous system — and that this opposing response grows stronger with repeated exposure.

The Basic Mechanism

1. A stimulus triggers a primary emotional state (State A)
2. The nervous system automatically produces an opposing state (State B)
3. State B outlasts the stimulus slightly
4. With repeated exposure, State B grows stronger while State A weakens

Drug Addiction Model

Initial drug use: intense euphoria (State A), mild withdrawal (State B).
After repeated use: moderate euphoria (State A has weakened), severe withdrawal (State B has strengthened). The person now uses primarily to avoid the intensified State B — not to experience State A.

Love and Attachment

New romantic love: intense excitement (State A), mild sadness at separation (State B).
Long-term attachment: moderate comfort (weakened A), profound grief at loss (strengthened B).
This explains why the loss of a long relationship is so devastating relative to the initial intensity of attraction.

Reference:

Wikipedia: Opponent-Process Theory

https://en.wikipedia.org/wiki/Opponent-process_theory