Initially, once the excess affection i.e love showering stage is completed, the narcissist start to lose the interest in the relationship. Gradually, the relationship no longer gives him the ego boost/ the narcissistic supply.
This is when the narcissists start devaluing you. This initially begins as a funny way which as days pass by turn into serious devaluations.
Days later, when the narcissist no longer cares about you, they start abusing you for each and every reason. As the narcissist is not getting the expected narcissistic supply from you nor the relationship, his/her thoughts of leaving you, start off.