Prussia was able to persuade other German nations that they needed to gang up against their common enemy, France.You want stability and less war, you need less small countries bickering over ancient grudges and territory.
But having one hegemon, and several independent countries, is not necessarily going to help.
Diplomacy only works if Bulgaria has something to offer there - its pretty hard to talk someone out of war with words alone.
Maybe Bulgaria can do the same with Turkey?