What caused Bulgaria to enter World War 2?

Bulgaria entered ww2 because of the treaty it had with Russia. Russia and Bulgaria were side by side and their treaty had not yet ended. as a result, Bulgaria was to some extent forced to join the war otherwise it would be under extreme threats from Russia
With the HOPE that it could restore its lost territory during Balkan wars and WW1. And to avoid San stefano treaty and Berlin treaty (Bulgaria thought these treaties were unjustified and imposed to it).