Why regret?

Why do we have feelings of regret?

In June I asked this to friends in an Instagram story and it’s still rooting around in my mind.

Why isn't every single decision rationalized away by our brain?

We rationalize lots of our "wrong” or “immoral" choices; why not all?

Including things like, "I wish I'd asked them out on a date back then", or "I wish I'd kept in touch with my friends."

I asked Robin Hanson and he quickly said a good point: "Others are more tolerant of our transgressions when we regret."

For sure, and Tyler Cowen said not to believe everyone on their deathbed saying they wish they spent more time with their family rather than work. Maybe they spent their time exactly as they wanted to, and even on their deathbed they’re following decorum…

But what about internal regret, purely privately felt?

If it’s a teaching tool (like you could say feelings of depression are, too) — is that the only reason we feel regret? Or is there more to it?