Why is Imperium of Man so mean?

Please explain this to me from the lore perspective. I'm playing the default human game. The long-lost brothers who flew to Deneb border me. They immediately declare rivalry, worsen relations, and soon after, declare war.

Meanwhile, on the other side of their empire they are bordered by driven exterminators (or whatever the name is) and the ones that want to vassalize everyone. Imperialists? I forget. And yet, these folks have nothing better to do than declare war on Earth that they came from 🤔