If someone gave you a perfectly reasonable and logically consistent explanation, what makes it right to automatically assume they're lying and substitute the worst possible alternate explanation?

The point isn't that you have to trust 100% of what everyone says. The point is that it's a problem if you're trusting 0% of what everyone says and using that as an artificial opening to insert your own, alternative explanation in which you are the victim.

A general predisposition to assuming a good nature of your fellow humans?

A notion that seems very culturally hidebound, rather than something that describes a general state of human psychology that policy should dictate that humans should be brought into alignment with.

No, it seems to be a universal truth that humans _generally_ cooperate, else we'd have all killed ourselves off (or been forsaken to eternal tribal squabbling, never able to coordinate enough to build a "real" society).

Yes, humans cooperate. But it's not a universal truth. Indifference is the default state of man's relation to man and it is man's prerogative to decide who should receive his consideration. With this in mind, it's better to say humans are contractual. They can cooperate when the incentives (or disincentives) align towards doing so. They can also ignore or attack each other when such incentives are ineffective or insufficient.