Experience is the best Teacher
There are some things that just can't be taught, you have to go out there and just experience it yourself, either firsthand or watching it happen to someone you know, only then would you really understand that thing and know how dangerous it can be.
I like to think of myself as an innocent soul, and this is because I tend to see the good (almost like I try to ignore everything at first and just focus on the good) in people when I first meet them, maybe a little bit too much. And I say that because it always surprises me how surprised I get when I find out something new about someone that contradicts what I initially thought of them.
It surprises me because most times, I don't even know that I held them in such high regard or that I think very highly of them, until they do that thing, then I find myself experiencing some sort of sadness, not because I feel bad for them and for what they did, but because I'm disappointed at the fact that they ended up not being the kind of person I thought they were.
And to be honest, I shouldn't always feel this way, especially in the world we're living in today, because there are a lot of crazy things happening today and people this days are no longer trustworthy as most of them don't care about morals and values anymore, they just do whatever they feel is best for them, even if it includes backstabbing someone close to them.
This is why earlier today, I was telling a friend of mine about how important it is to always observe people and their behaviour. I told him that even if that person is nice to him, he should observe how that person treats other people and make sure to have contingency plans in place, just in case he finds himself being treated the same way tomorrow, because if they can do that to someone else, they can do it to you too.
Unfortunately, advises like this usually comes into one ear and leaves out the other because things like this are usually better understood after having an experience with it. I've had my experience before, luckily for me it wasn't a bad one, but rather an eye-opening one, and now I find myself being a close observer of almost everyone in my life.
Hmm. Nobody can be trusted fully because there are many people who want to take advantage of us. As you said, it's good to observe how one person treat others to understand the nature of a particular person.