Trust is different from expectations. I expect a lot of things to happen. I expect that strangers on the street won't try to punch me in broad daylight. I expect that I will have a job tomorrow (and next week and next month). But that doesn't mean I will let my guard down. There is always a chance that somebody would decide I was a good punching bag. There's a chance my job might not exist tomorrow (or that it would exist and somebody else would have it). In either case, I'm prepared. There is always a plan, a recourse.
If my friends decide to blab my inner secrets, I would have zero recourse to protect myself. If my friends wanted to rob or murder me in my sleep, they could. But the reason I don't worry about it is that I trust them not to. And this is what I mean. Trust is the ultimate power, because it is another person giving you control over them.
For this reason, I believe that betrayal is the worst atrocity one can commit upon another. Betrayal is the violation of trust. It is the deepest wound one can inflict and it is permanent. Nothing can ever truly repair betrayed trust.