Numbers. Your psychopathic neighbors who want to steal your stuff are often rallying under a single banner, their violent government. The nature of war is such that defending yourself in battle requires large scale coordination to be most successful. Thus you're going to need to rally your own people under a single banner to successfully defend yourselves.

This is exactly why almost every neighbor of Russia has either joined NATO, or is actively trying to join. NATO is the biggest banner they can rally under to discourage Russia from invading.