What do you think about the concept of "trust"?

I've always found the concept of "trust" particularly interesting from a psychology standpoint. You often hear people in American society say, "I trust someone until they give me a reason not to trust them."At least they used to say that - perhaps people don't really say or think this in America any longer due to the racial division and problems. However, I remember as a kid hearing this quite frequently from people. And even Erik Erikson's early stage of development is trust vs. mistrust - as if it is "all or nothing" or black and white. But is it really? Is it really that we trust someone 100% and once we trust someone we always trust them? Or if we mistrust someone then we always mistrust them? What if the person we said we initially gave our trust to because we feel we should trust someone until they've proven they can't be trusted, then violates our trust? Will we forever no longer trust them?