You have answered your own question. When we sin, we hurt not only God but others as well. Like dropping a stone into a pond, sin has a ripple effect. The Catechism of the Catholic Church summarizes this very well.
1456. "Confession to a priest is an essential part of the Sacrament of Penance: 'All mortal sins of which penitents after a diligent self-examination are conscious must be recounted by them in confession, even if they are most secret and have been committed against the last two precepts of the Decalogue; for these sins sometimes wound the soul more grievously and are more dangerous than those which are committed openly.' [Council Of Trent (1551): DS 1680 (ND 1626); cf. Ex 20:17; Mt 5:28.] When Christ's faithful strive to confess all the sins that they can remember, they undoubtedly place all of them before the divine mercy for pardon. But those who fail to do so and knowingly withhold some, place nothing before the divine goodness for remission through the mediation of the priest, 'for if the sick person is too ashamed to show his wound to the doctor, the medicine cannot heal what it does not know.' [Council of Trent (1551): DS 1680 (ND 1626); cf. St. Jerome, In Eccl.]"
What the Catechism of the Catholic Church says on "Penance:"
You know, I was re-reading some of your posts on this thread and one of the thoughts I had was how beautiful and simple Scripture is compared with all of these quotes you keep posting from your Catechism.
Instead of clarifying Scripture, the Catechism you cite is so wordy and cumbersome that it reminds me of how a bunch of lawyers can sit around and make something simple complicated to the point that only they can understand it, or at least claim to.