Serving as a judicial officer in universities for much of my early career, I've got pretty clear standards of when something is (and isn't) proven. Given that I can be a suspicious creature, such tempering is a very good thing. As a result, I know that, most of the time in my interactions with others, there's simply no way to justly assign motives and agendas—there's so much we don't and can't know. That leaves lots of blank spaces, and Amy's rule is a constant reminder of what I'm called to do with those spaces.
It's staggeringly damaging to do anything else. Almost any other way I fill in those spaces tends to employ darker stuff, robbing others of the dignity we're called to afford them. I've been a giver and a recipient of this, and I can attest to the damage either way. Personally, the hardest part is this: even if (in the absence of evidence) my believing less than the best of others is right, it is still likely not good (evidence, of course, calls me to a different course; with other Christians, Matthew 18 provides the guideposts for that path, which is also characterized by love and grace). God alone defines good, and this is the way He's called me.
"I don't know" is a tough truth for many of us to admit. Pride can no doubt be a part of the difficulty, but I wonder if in some ways we're simply (though often deeply) disturbed by the blank space such an admission opens. With others, I have to challenge myself to expand this statement to: "I don't know, so I'm called to believe the best." This isn't naive, but it is vulnerable. In love, I take up a position where I must trust Someone Else to be my defender and protector should I need one. He's worthy of this trust, though, and learning to trust Him more is worth anything else that may be at stake.