Why is Stamboliyski's Bulgaria attacking Serbia?

In he beginning of my Serbia game I got a message about this guy being a friend of Serbia and supported him. After I beat Bulgaria the 1st time he came to power. But then he started the usual Bulgarian revanchist shit, attacked Greece, didn't even finish it and attacked me. Naturaly we destroyed Bulgaria again. Was there any way to befriend him instead? Was he aggressive because I took RadSoc path and he is an AuthDem?