In other words if they see harmony and love it engenders a responding love in them that wants them to become grown ups like their parents. This is the main leverage parents have in controlling or motivating their children. If parents don't have this reciprocal tie with their children they are helpless. There is nothing one can do if they are defiant as it is seen in many psychiatric clinics. Some people think of love as meaning simply the expression of physical and emotional affection.
Parental love is multi dimensional.
It's wanting the children to grow up to be responsible citizens and successful individuals. It is reminding them everyday how to behave to become that kind of person. There are many aspects to managing and motivating children, the example set by parents, sincerity amongst the parents, their consistency, their self assurance, their respect for children and their manner of punishment. But the unconditional love for their children and the responsive love of their children for them is by far the most important.