Chapter 2 argues that people do not learn love fully in flat households. They need relationships above, beside, and below themselves. That is why grandparents, parents, and children belong together: they teach love across time and direction.
Three generations let a person inherit the past, serve the present, and invest in the future in one place. Without that range, love gets thinner and more abstract.
This makes the multi-generational family more than a cultural preference. It is a formation structure for learning how to relate to the whole human world.