Money. Soldiers are paid well. But there is an ideology. Russians (not in the sense of nationality, but residents of Russia) want to defend (from Kiev's aggression) Donbass and "liberate" all "Russian cities" from Western influence. Ideally the entire coastal territory of Ukraine up to Transnistria. "Am I a trembling creature or do I have the right?"

Ukrainians believe in the European world, want to dictate their will in the southwestern region and receive benefits from the west for it. In fact, independence is out of the question here, only the choice of a suzerain. And they have already made this choice. 

 Allright thanks good answer as i was expecting from you, but what about the first army thing?
And i forgot before to ask you, do you think that kicking off the NGOs out of russia and putting navalny, the western puppet, in jail is undemocratic or protection of democracy?