To start with, I'll just lay down the points of discussion, numbered to make them easy to refer to, later in the post...
1. To trust is to be vulnerable.
2. To trust blindly, is to ask for trouble.
3. If you trust someone, you give him/her the power to hurt you.
4. When you trust blindly, there's much higher risk that someone you trust will actually hurt you.
When you trust, there are two main elements involved: the guarantee that you're expecting, and the guarantor. There has to be a close tie between these. (Please assume for the purpose of our discussion, that the guarantor doesn't explicitly communicate that he provides the guarantee. Also assume, that what you're trusting is something positive. Not, for example, "I trust that the bus will be late.")
If the guarantee is delivered, we stand to gain, since things happen the way we hoped for. Otherwise, we stand to lose. This justifies point 1 above and, partially, point 3. I'll come back to the reason for saying "partially" later in the post.
Now, what are the situations in which the guarantee is not delivered? Maybe the one we trust can't actually do what we expect. Lets say, I trust my friend borrows my Ipod, and I trust him to return it in the same condition, but he returns it with scratches. Or I trust a friend to reach on time for my birthday party, but he gets stuck in traffic and reaches when the party is almost over. Possibly, the only way to handle these scenarios is to develop a kind of inbuilt analysis of whether someone can be guaranteed to do something. It's still not really a guarantee, though, because there could be a lot of unknowns involved. When I understand that there isn't a high certainty the event happening, I plan for the non-occurance mentally, and also try what is known as "risk mitigation". If Alfred stays on the other side of town, and I know he can't start from office earlier than 6 PM, there's a chance he'll get stuck in the traffic on the way, and I should be mentally prepared to move forward with the party, and also put aside some cake for him so that it's not all over when he reaches. In other words, I trust that Alfred will "do his best" to reach my party on time, but may not be able to. This is where I say point 3 is partially true. Agreed, Alfred may not "do his best" and may get hooked to some girlfriend on the way, and hence be late. But if I know Alfred is inclined to do that, I probably wouldn't be surprised (or "hurt") when I find out. :)
When we trust "blindly", we just presume that everyone will understand the implicit "guarantee", sometimes even when there's no scope for such common context. Whenever we hear "I expected him to do X, but he didn't" ringing in our minds, and feel hurt, the things to check for are: Does he know that I expect X? Is it feasible for him to do X? Another important question is: Would he want to do X? Lets suppose you're travelling alone in the train with a lot of luggage(generally a bad idea, but anyways...), and you want to take a quick stroll down the compartment. Now, you might have been chatting a lot with the nice family that sits next to you, and might be tempted to request them to "watch over" for a few minutes. And they might agree too, but they might not really be in much of a position to take care of your luggage, or even want to do so. If someone picks something out of the front pocket of your bag, they might not even notice, and it might be in your best interest to lock up luggage, chain it, and then take your stroll. Of course, the new assumption here is that someone cutting chains or breaking locks is quite likely to catch other passengers' attention. :)
Another time when we tent to trust "blindly", (and the more common one) is when we have known them for a long time. Most of us trust our families blindly. Sometimes, this does crop into unfulfilled guarantees("Dad, why didn't you buy me an Xbox for my birthday?" :)), but we don't "lose out", since we know it was just a misunderstanding and not an ill intention. We talk to them and clear out the misunderstanding. Also, one could argue this is not blind trust, and instead a relationship built over time, where we know how they are likely to react to almost any event.
We also tend to develop a lot of trust for friends that we've known for a long time, and expect a lot of unspoken understanding, just like we did with our parents. There's always a likelihood that this kind of trust can go "blind" at some stage, where the friends keep making assumptions and not communicating. And when that happens, the chances of your friend hurting you can actually go quite high, because the expectation of "guarantee fulfilment" is very very high... sometimes because you didn't understand the friend's intentions right, sometimes because he didn't know you had the expectation.
In short, don't trust blindly, and you'll be much better off. Don't trust anyone for anything and everything. Make sure you trust them to do the specific thing you expect from them. If unsure, try to make sure somehow, or maybe lower your expectations.
