Faith in Humanity is trusting that most humans have good in their hearts. That most of us, though we are frail and faltered, have good intentions. We, as humans, strike out with the best intentions. Therefore, we can trust, though error, most of us are trying to not do wrong, do to you wrong.
It can feel naive. I feel naive, at times. But I like my life better, living it this way. These same beliefs I hold myself accountable to as well. Though I may make mistakes, hurt feelings, etc., my intentions are not of an evil or conspiring nature. With that I can find some peace in life.
Of course, I do believe there are exceptions to that rule.