People constantly generate self-fulfilling prophecies. You do it whether you want to or not, so you might as well harness this for good.
If you meet someone that says “People, as a rule, are selfish jerks,” then that person will be proven right again and again. But they’re creating the conditions for this; their belief affects their demeanor, their demeanor affects their choices, and their choices affect how people interact with them – and even which people are around to do so. In other words, they’re callous and mean, so the people around them become that way, too.
If you say “People, as a rule, are kind and thoughtful,” then you’ll act accordingly. You’ll make it easy for people to be nice to you, and you’ll generally end up surrounding yourself with kind and thoughtful folks.
The world is full of people of all stripes, so no statement about them can be universal. But it can be true of your world if you make it so.
So, decide for yourself what you want to be true, and then act as if it was. Even if it only nudges the world in that direction, that’s better than the reverse.