We might not think about it too often, but a single person's behaviour can change someone's day, for better or worse. You can smile at a stranger who's just lost a family member and they might see light at the end of the tunnel. Or you can frown at a stranger who's just gotten engaged and they start to think of all the reasons why it wasn't the best decision of their life, without even noticing it themselves. If everyone was nice to each other, or at least tried to, this world could be a better place. Putting others above yourself every once in a while would probably decrease the number of people with mental issues, help developing countries and just lift the mood on this planet in general.
Double standards and hypocrisy are a couple of the major issues while discussing this subject. Some people seem eager to tell others what to do and are saints in theory but when it's their turn to do some good they often let their true colors shine bright. I'm not saying I've never done this. I might be one of the worse hypocrites existing. I sometimes notice I criticize people a lot in my head and then end up doing just like them. But lately I've really started to give some thought to this.
When I'm about say something about someone I try to stop for a minute and think - is this something I could say to their face? Also I think judging people based on their appearance is total bullshit. If someone has done something unacceptable it's a bit more understandable and even then - who knows what's behind their situation? So I'd say judging people in general is quite a big load of bullshit. If you have an unbearable desire to do so, at least keep your thoughts to yourself.
It's not just about not being an asshole towards other people, although that alone is a great improvement to some of us. It's also about being nice. Flashing smiles, giving compliments. Of course this isn't necessary to do if you absolutely despise someone. But your friend can be having a terrible day and you telling them their hair looks nice could make it all a bit easier. Don't forget those closest to you.
There's an eternity of things wrong in this world and good manners are something anyone can do to make it all slightly better. Being nice to everyone isn't two-faced, it's called maturity. It doesn't cost you anything either, so why not try? Making others happy often makes you happy too. Mind your manners.