We talk a lot about trust in relationships. We trust our partners when they tell us their feelings. We trust them about who they are interested in, why they like us. In order to feel our most profound emotions we need to open up, to trust each other. At least that's part of the story.
I'm not sure that part's true. We can feel all of those feelings—love and excitement, passion and interest—without trust. We can even feel another emotion that isn't fun for everyone, but is part of most relationships anyway—fear.
Of course, some of us thrive on fear. We thrive on either evoking it or experiencing it. We thrive on the thrill of potential rather than trust in the known.
Trust, however, is the safety net. Trust is the emotional condom to stop the infection of real (or nonconsensual) danger. We find people who are worthy of trust so we can relax and experience the emotions we want to.