Thoughtfulness costs nothing. I prefer to call it that rather than 'manners' which sounds more Disgusted of Tunbridge Wells! But call it what you will - we brought our daughters up to be thoughtful and considerate and they are, as far as I can see. That means 'do as you would be done by' too, and as a family that's what we're trying to teach the grandchildren.
However...I have to confess that I've not always been considerate. It took DD1 to point it out. Depression and anxiety meant I had a short fuse and was pretty grumpy which of course has an effect on other people.
Now recovered, I make an effort to curb any grumps, smile and think of other people who have just as much right to 'be there' as I do (for example). Sorry, badly put, but you know what I mean!
Result: you find yourself chatting to people, seeing smiles, having thanks or help reciprocated, and it makes everyone feel better. Just pay it forward, as they say. Or try to do so. Life's too short not to.