Why were kings from other countries worried about the French Revolution?

The "coalition" kings were afraid of

a) a "domino effect" in their own kingdoms. They feared that their people may uprise just like the French did.

b) war and revolutionary France's military ambitions. By 1789, France was the most populated state of western Europe and gathered a huge number of benevolent soldiers to its borders to secure the young Republic and its zones of influence.

c) Commercial and political instability resulting from a change in the "balance of power" that kept a fragile "statu quo" ruling Europe.