An unpractised observer expects the love of parents and children to be constant and equal.
 Samuel Johnson, The History of Rasselas, Prince of Abissinia (1759). copy citation

Context

““In families where there is or is not poverty there is commonly discord. If a kingdom be, as Imlac tells us, a great family, a family likewise is a little kingdom, torn with factions and exposed to revolutions. An unpractised observer expects the love of parents and children to be constant and equal. But this kindness seldom continues beyond the years of infancy; in a short time the children become rivals to their parents. Benefits are allowed by reproaches, and gratitude debased by envy. “Parents and children seldom act in concert;” source