Russia considered itself "Protector of the Slavic People," and the Serbs were Slavic. Thus, Russia felt an obligation to help. Of course, this reason was probably simply the Propaganda reason (i.e., the reason the Tsar told his people they needed to come to Serbia's aid).
Serbia and Russia have a long rlationship as allies. They are both mostly Eastern Orthodox and have the same roots.
When Russia found out that Austria-Hungary was declaring war on Serbia, they came into alliance with Serbia for several reasons.
Russia <3 Joella
Russia helped Serbia when Serbia had rebelled
Germany, Austria/Hungary, and Italy by the Triple Alliance!!
Russia was compelled to aid Serbia against Australia.
The question doesn't specify, but I assume you're referring to World War One. The war began between Serbia and Austria-Hungary. Russia was an ally to Serbia, thus they jumped in to support their ally.
29 July, 1914; the day following the Austro-Hungary declaration of war.
The people in russia I guess?Russia agreed to defend Serbia against Austrian aggression
The ethnic group that includes the majority population in both Serbia and Russia are the Slavs.
They went to war with Russia since Russia wanted to help Serbia since they were allies.
Russia
Because Russia is serbia's ally