We all go through millions of emotions in our relationship, and those emotions create brain chemicals that change the way we feel. Sometimes we are in a positive frame of mind (hopefully most of the time), and other times we can be neutral or even having negative feelings about our partners and ourselves.
Being able to trust your lover with your feelings is part of having a great relationship. When you share something personal—perhaps that you are anxious about something at work—and your partner is supportive, it builds trust. It also gives you more strength to deal with whatever your issue is.