I realised something lately.
There are precisely three people in the entire world whom I actively dislike. I won't name names, but they probably know who they are. And all three of them have earned their place in that list by abusing my trust in one way or another.
Now as a rule, I'm not one to bear grudges. Of any sort, really. You can wrong me in pretty much any way you like, and (perhaps given time), I'll let it pass and forgive and move on. I guess it's down to some sort of sense of “nobility” and it being the Right Thing To Do. But abuse my trust, however, and I simply can't forgive that. That's what I've realised. I never really realised just how much value I placed on it before.
I'm not naturally a trusting person. I don't tend to confide in people or rely on people all that much. So when I do, it means something. And if someone willfully betrays that trust, or lies to me, or anything that fundamentally undermines the confidence I have placed in them, that's when they end up in a place from which there is little or no redemption. I can forgive pretty much anything else, but not that.