[consteval]

> what existential reasons do we have to behave well?

Overall humans play pretty nice, all things considered. If you get sick someone will help you, if you lose your job you can fall back on assistance. Hell, even the fact you HAVE a job is a privilege granted to you.

Other animals don't do that. They eat their young, because they feel like it. Without the will and kindness of other humans, a fact that we take so for granted we fail to recognize it as "society", most of us wouldn't live 10 days. That's not an exaggeration either - check out infant mortality statistics throughout human history.