Trust? As everyone else said, you can't trust others 100%. But when you show vulnerability- you get some rewards (like being in love.) But, when the relationship is done, the trust seemed stupid, didn't it? It's tough...
I'm pretty sure your friend showed signs of envy or whatever it is she/he did before they acted upon it. You just didn't read it off them. Things people say and how they act around speak volumes - even if they are trying to hide something.
Whenever you hear somebody say something like a subversive joke at your expense, or something that "cuts you down" but is said in an off-hand "inoffensive" way (you've heard stuff like that) are signs that there's something going on underneath. You need to be more aware of people's actions and what they let slip in conversations as well.
Some of the best readers of people are not the "educated" social-ed majors, but hustlers that grew up on the streets. They are people that understand that actions mean a lot more than words. Also, they are more attuned to undercurrents in people's statements - it's a survival tactic that is honed over the years.
Try to sharpen your instincts and read a little better into people to see them the way they truly are. An example is someone you know that may call you up all the time, say they're a great "friend" but rarely visit you. You visit them more than they ever do. That's where actions speak louder than words.
Reassess your relationships and try to grasp what they REALLY are, not how you WANT them to be.
Once you understand how to trust your natural instincts better (that gut-feeling) and honing your people-reading skills, it's like you're in the fucking matrix. You see things a little differently, and that new "view" actually helps you be a better person.