Who counts? Why do governments deny secession in some cases but not others?

There is a received wisdom that states will always deny secessionist demands. Land is too valuable and/or too important to the national image. The very idea of the sovereign state is predicated on territorial control. Permitting one region to secede will only embolden others; therefore, the links in the chain must be defended. These are all commonly given explanations for why states will deny secessionists and fight them if they have to. There is no question that blood has been spilled over the issue, from the US Civil War to the conflict in Biafra to the fighting in Chechnya. It is estimated that half of the civil wars since 1945 have involved secessionism (Griffiths, 2015: 733), and one prominent scholar claims that secessionism is the chief source of violence in the world today (Walter, 2009: 3).