I know we can not trust anyone 100%, because people will let you down.
But one thing I have learned over the years is to give people the benefit of the doubt until they have proved that they are not good.
I am not saying to be a fool, but there are people in the world who are honest, who truly care, who want the best for others etc. You can not live your life being suspicious of everyone. It's about having good eyes. Choosing to see the best in others.
I don't care what anyone thinks, I have been messed over a lot in life, but I still choose to see the best in people.
I have been traveling a lot around the world recently, and I have meet many nice people. People who are truly good at heart (please don't go religious on me and start quoting "no one is good....").
There are people who don't even know you or have any obligation to you, but they treat you like you are someone worth caring for. And it shocked me sometimes. But this is a fact. I have meet many people that shocked me. They were so naturally kind and caring. And of course I have meet other really awful people, but I can't judge the rest of the world because of them.
In a nutshell, give everyone a fair chance.