You have to adjust your priors at some point though, right?

    Fool me once, shame on my prior.
    Fool me twice, shame on my posterior.

Adhering to that adage eventually becomes something masochistic and Sisyphean.

Apply it to your personal relationships, but you will be steamrolled over and over again if you naively assume good faith in politics, business, etc even after you've been flattened to a pancake.