Trust is about how we deal with uncertainty. Even when I have proven myself trustworthy to you time and time again, over many years, there is always a remnant of uncertainty. Heck you aren't even completely certain that you can trust yourself!
Our ability to choose to trust another person is an emotionally driven choice. Paul Zac's neuroscience research into what makes trust in the brain clearly demonstrates that the neurochemical oxytocin is a precursor to trust by the conscious, deliberate actions.