I think by the time you’re 40 you have enough data that you can spot patterns, and it’s easier to identify a dickhead-in-waiting before they have the chance to prove their dickheadedness beyond reasonable doubt.
My theory is that people who get along easily with everyone don’t have very well-developed spidey senses, and can’t tell when people around them are unstable or antisocial in subtle ways.
Possibly because they grew up in safe and secure environments where you didn’t have to start from the assumption that most people around you were volatile or dreadfully moody or a bit insane, and work out how not to trigger them.
But once you’re familiar with the little signs that someone has it in them to be mercenary or controlling or manipulative or sadistic or just a plain old emotional vampire, you can’t unsee them.
I don’t think most people are dickheads, as it happens. But just as I can spot dickheads, they instantly recognise me as someone who was raised by dickheads, and groomed to meet their needs, and so they make a beeline for me wherever I go.
So I tend to keep a low profile! I stick with the few people I am 100% certain are not dickheads.