"Most psychologists and other social science researchers regard trust as just one of many qualities that determine a relationship's strength, rather than its foundation. Some exerperts even consider trust a character triat - you either have it in you or you don't. But I don't believe that. I am certain that the majority of couples can maximize their loyalty level and therefore guard against betrayal and improve their odds of a happy future together. Trust is not some vague quality that grows between two people. It is the specific state that exists when you are both willing to change your own behavior to benefit your partner.
"Trusting each other doesn't mean that [a couple] will always put the other's needs ahead of their own - that is unlikely to be healthy. But it does mean that their happiness will be interconnected. They will each change their owwn behavior to increase the other's payoffs.
-John Gottman in What Makes Love Last?