Trust builds slowly as we pass time with our partner, and they become more predictable to us. Predictability is important because having an idea of what will happen makes us feel in control of the relationship and our lives. As we observe how our partner thinks and acts in some random situation, we develop a sense as to how they will think and act in any future situation.
If they are consistent with how they act and seem to be sincere about having only the best interest for us, we can relax and believe that they will continue to do so in the future, so we can trust them. There's an element of faith operating with trust because we really can't know how our partner might act or say before the fact. Having faith in your partner, meaning you believe they will do right by you before they do it, is an indicator of a good relationship.