Trust. With only five letters, it has a lot of meaning weighing in behind it. It's one of those words that is deceptively short for the amount of meaning there is behind it when you say "I trust you."
Of course, we've all been hurt by someone we've trusted too much and they've done something which broke our heart or trust. It may have taken days, weeks or months to heal from it. Some people carry that heartbreak with them for years afterwards, pushing away anyone who gets close. Inevitably, it all boils down to the same question: since it hurts so much, how do I avoid getting hurt by someone I trusted?
The most obvious solution to most people is to treat our trust like it's locked inside a fortress. Any time someone approaches who we might have to trust, we then start agonizing over whether or not to trust them, how much to trust them, and what if they betray our trust. There may be difficult protocols to get in, an extensive waiting period of examining a person before they can be trusted, or in many cases outright refusal to trust based on a trivial piece of information.
Of course, this never guarantees perfect security. Most high-security prisons have been escaped from in the real world and countless high-security facilities have been breached. In our modern high-tech world the single greatest threat to the security of these facilities is social engineering - the art of manipulating people to get a piece of information that would be difficult to obtain by conventional means.
Keeping people are arms length opens up what I see is a critical flaw in this system - you have no basis for how trustworthy anyone is. If someone you know approaches you and asks for your trust, you have no history of their trustworthiness because you've never had to trust them before. You are faced with the critical choice of trusting them with almost no useful information and resort to irrelevant information as the basis of your trust ("well he's kinda cute, so I guess I can trust him.") In essence, despite the fact that you've resolved to be a fortress against untrustworthiness, anyone with reasonable charisma can approach you and have fair odds of being let past all that security unchecked.
The biggest problem to me is that all this Fortress Trust stuff focuses too much on the negative aspects of trusting someone (the possible future breach of trust) at the cost of ignoring the many positive aspects of trusting. By keeping trust locked away in a tower, that very human connection of having someone to confide in is lost. If trust was just a game about waiting to get hurt, we wouldn't play it.
So my conclusion is ... the trust must flow.
I believe that a better way to deal with wondering about who to trust is turn the situation around backwards. Instead of starting with trusting no one, start with trusting everyone - a little bit. Anyone who comes in contact with you is immediately given a small slice of trust pie, paid up front. Discard the old adage that "trust is earned, not given" and be the first to offer a reasonable amount of trust.
This of course doesn't mean you immediately entrust your life, worldly possessions and identity in the hands of a total stranger. Start with a small piece of information or a favor. Confess a hidden flaw, a dream, or a fear you have. Let them see a small token of your trust in them without them having to ask for your trust first. Essentially, take control of the situation by issuing trust first.
Once they have received the largesse of your trust, you are able to see how they react to being trusted. Some people enjoy repeating privileged information to others. If you have confided in one of these people, your information may soon make it back to your ears, and you will have been able to infer their truthworthiness. Some people enjoy using information as a weapon against the people involved. An untrustworthy person may soon turn that information back against you or try to exploit it.
If the person repays your trust with some of their own, then you can pay back into the relationship and reap the benefits. If they do not repay the trust immediately, give them time or offer more pieces of trust. Do not prompt them for their trust - this will make them defensive or make them unwillingly divulge information. If they are just a sponge for your trust, then quietly draw your trust back from them. If they anger at the lost trust, then this is also a poor sign. If they don't even notice the trust evapoarate, then they are simply indifferent to your trust and so it would be wise not to waste it on them. Do not throw pearls before swine.
This ebb and flow of trust, mutually shared without reprimand, must be the ideal state of a relationship. It makes building a knowledge of someone's trustworthiness immediate and instinctual. By being actively involved in granting trust, it becomes easier to recognize people who are making obvious plays for your trust, as well as those who will not respect your trust. Lastly, by having multiple people you are actively engaged in trusting, even if someone betrays you, you are never cut off from your only source of trust.