Valis and other's make a good point in that a mediator is absolutely necessary. Hopefully with a lot of objectivity.
One good reason was mentioned by several people:
'Dave feels as if Kammy loves pot more than her'
This is a BOGUS issue, but if this is what Dave and Kammy argue over constantly, their relationship is in big trouble.
Why?
Love is a drug. This is very very real here folks. Drugs are powerful because they bind to receptive areas in the brain which are normally for control of bodily sensations and/or behaviors.
We are constantly driven by impulses from psychoactive drugs, whether or not we are addicted to drugs synthesized outside our bodies.
Dave will try to administer his drug of love to Kammy in an attempt to override her impulses for other drugs and wean her from her addictions.
Psychiatrists are licensed to administer drug therapy. Not Dave. He is defeating himself.
Dave cannot continue to act as if his love can 'cure' her.
What should Dave do about the relationship?
Remember that Kammy is Kammy and Dave is Dave. Kammy is not Dave.
If Dave's love (= care, concern, apart from attraction and desire) is taxed too heavily, then he should redefine his relationship with Kammy. This is the real issue, not all of that ego nonsense.
Will his ego stand for being nothing less than the most important thing in Kammy's life? That makes for an unhealthy definition of an intimate relationship.
"All that is necessary for the triumph of evil is that good men do nothing." Edmund Burke