The Narcissist has no real identity, only an illusion of themselves built on their ability to control other people. When they lose control, this illusion is shattered. This explains why they shift into turbo gear when you implement No Contact. No Contact makes the Narcissist feel small, worthless, and powerless. They whip themselves into a frenzy because they want your supply to maintain their false image. When you go into No Contact mode, you are shifting the power to yourself, and the Narcissist loses not only their God-like power, but also their emotional punching bag and source of narcissistic supply.
Narcissists are never truly happy and content. They are full of rage, insecurity, and hatred. Due to this, he has a load of vile bile stewing in their stomach cavity at all times, and they need someone to projectile it onto.
Think of how you felt after being fed upon by the Narcissist and discarded. You’ve been emotionally, physically, and/or emotionally abused and cast-off like a paper bag from McDonald’s. You suffer from PTSD, Trauma Bonding, Stockholm syndrome, crippling self-esteem, and depression. The reason you felt this way is not only because of how you were treated, but because you’ve effectively internalized the way the Narcissist feels about you. This reason they must devalue and discard their victims is because once they have destroyed you completely they can't feel good by destroying you more, and besides, the Narc needs to find a new receptacle for abuse and narcissistic supply.
It’s also why they appear so happy with the new supply. The new supply doesn’t know the Narcissist’s agenda and will go along with whatever the Narc asks of them. But as was the case with you, and all the partners they have ever had, the new supply will eventually be devalued and discarded, too.