One of the most important foundations of a healthy marriage is honesty. By providing sincerity in the relationship, a partner affirms his or her level of commitment and dedication to the other. Honesty proves that the partner is dedicated, and allows building of trust.
Trust is of course paramount in any marriage. When trust is lacking, the partner always wonders about the other, and sometimes such wondering leads to imagining the worst. When that happens, the desire to remain attached and committed is diminished, and the marriage gets into serious trouble.
One aim of a psychologist or therapist is to communicate such importance of honesty, and giude the couple back into a trusting relationship and ultimately a happy and long lasting marriage.