Manners.
Back in the day they used to be, especially when wielded by the British, a form of enforcing social conformity by picking on the smallest detail and whacking someone over the head with what they did not do. Briefly, after WWII, they also became a way of helping to ensure dignity among members of society in an attempt to create an environment where what happened then wouldn't happen again.
Then, sometime in the last few decades, they got thrown out with the baby and the bath water.
But without a common set of fall-back behaviours to ensure smooth interaction among people (and to provide dignity) we are left with our own methods of ensuring this. Which, if you grew up and were either taught manners or you figured them out, help you get fairly far in life. If you didn't, well, you're left making things up and, in some cases, they are subject to your overall mood. Which can cause negative feedback loops.
If you're having a good day, you're more likely to greet people with a 'hello', say a 'thank you', or smile; this causes a positive feedback loop. If you're having a bad day, you're more likely to do nothing or be grumpy, which causes a negative feedback loop and drags you down even further.
Add to this that when you can't predict if someone is going to say 'hello' to you, or say 'goodbye' to you, you become sketchy and nervous and less likely to initiate or respond. Which also creates a negative feedback loop for the group or society in general.
So, on that note, manners can be very important. Not the stiff, you-must-do-this manners of the past, but the basics for getting by interacting with people in daily life. Sort of like falling back on professional behaviour at work.
So, when you meet someone acknowledge them. Say 'hello' or smile. From there you can comment or begin a conversation depending on how you feel and the vibe between you and the other(s).
When you leave, say 'goodbye' or 'thank you'.
If someone does something for you, say 'thank you'. If you need their help, say 'please can you ____' or 'excuse me'.
That's really most of it. The rest are more of a personal choice. But these are the ones that will help make the world go round in a positive feedback loop.
