Here’s why. Recognizing the unilateral declaration of Kosovo’s independence from Serbia legitimizes the doctrine of imposing solutions to ethnic conflicts. It legitimizes the act of unilateral secession by a provincial or other non-state actor. It transforms the right to self-determination into an avowed right to independence. It legitimizes the forced partition of internationally recognized, sovereign states.
It violates the commitment to the peaceful and consensual resolution of disputes in Europe. It supplies any ethnic or religious group that has a grievance against its capital with a playbook on how to achieve its ends. It even resurrects the discredited cold-war doctrine of limited sovereignty.
Unless there's a vote on the matter, any secession is morally if not legally invalid.