Your answer would be Russia refused Germany's demand to stop mobilizing its military after Austria-Hungary declared war. Russia had an an incident like this in 1908, and didn't want to go through it again. If they didn't solve the problem, Russia's allies wouldn't trust Russia no more because they would've demonstrated no protection towards them, and think that Russia's promise to protect was a joke. Russia was an ally of Serbia since they were the ones that helped Serbia to become independent from the Ottoman Empire.
