I recently listened to a Ted talk by Helen Fisher & she was explaining dopamine receptors. How these receptors are triggered during the "falling in love" phase.
From what I've read about meth, these receptors are shut down & even during recovery they can take over a yr or 2 for them to return. So a feeling of anhedonia takes place with depression & other MH issues.

I've wondered if the person I know that uses meth heavily purposely sought out a new relationship to trigger the dopamine high associated with it. Maybe not consciously, but the philosophy makes sense to me.