Perception is reality, reality is perception.  If you perceive something like your spouse cheating on you the facts of it being false becomes irrelevant.  I trust my wife with my heart and my life.  I know she would not cheat on me.  Her faithfulness I do not doubt.  I see friends and coworkers struggle with jealously and I wonder if somehow I just did not get that gene?  Perhaps I am the fool or too self assured that my reality is I can trust her.  Until I perceive myself as the fool I guess there is nothing to be worried about.