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.