If you use the "innocent til proven guilty" principle in your day to day interactions you're bound to get fucked by every 2nd person. Well, maybe not that bad but you will still get fucked because you don't have the same resources as a court does to figure out if someone's fucking with you or not.

You just don't have access to tax funded investigators working for months to figure out if the other person tells the truth or not.

So it's down to: Something's off? I'm not trusting you. Especially when you want something from me.