Why did Roumania Greece and Montenegro enter the war on side of Allies?

Romania, Greece, and Montenegro did not enter World War I on the side of the Allies. In fact, Romania initially declared neutrality, but later joined the Central Powers in 1916. Greece also declared neutrality, but eventually joined the Allies in 1917. Montenegro was the only one of the three countries to enter the war on the side of the Allies, doing so in 1914.