Isaac Arthur has a wonderful Youtube science channel and he makes an interesting argument in one of his Fermi Paradox videos: if a technologically advanced civilization discovers a nearby civilization, the probability that there's another nearby civilization(s) dramatically increases.

What that entails is, upon discovering a nearby advanced civ, it's now a reasonable possibility that everything you're doing is being observed by some other nearby alien civ that is potentially much more powerful than you. This is going to constrain a lot of aggressive behavior. Planets are sitting ducks, and the last thing anyone wants is some alien civ hitting them with an asteroid going half the speed of light, which is exactly what could happen if you don't play nice with your alien neighbors, so that's to be avoided at all costs. So, if you think you could be playing to an unseen audience, cooperation and passivity are the best strategies. Anything else is liable to bring on the asteroid/comet/projectile swarm of death.

I'm summarizing it and I probably left a lot out, but that's the basic framework. I think it's an original good argument. Thoughts?