Children can see through a parent who is made of muddy water. If they see disappointment and criticism, they will answer in kind. Discontented parents breed discontented children. Your attitude is the root of the family attitude tree. A bitter root cannot produce sweet fruit.
Parental attitudes are highly contagious, and children usually come down with a worse case.
More is caught than taught. And children seem more highly susceptible to catching a bad attitude than of being taught to have a good one. They can catch the disease of bad attitude while being passive. On the other hand, they must exert themselves to have a good attitude. Our actions speak louder than our words, but our attitude screams louder than our actions.