I found the definition of love on page 49 of my copy. I am using the 2007 hardback edition of the book published by Alfred A. Knopf in New York. Although page numbers may not be the same in different editions of the book, this is the best I can give you, since the book has no chapters.
It is actually not the father who defines love, but the mother. She tells the father what love is, and how it is the essential element which will make him keep enduring in the face of all odds. She tells him that, unless he has someone or something to love, there is no way he can even hope to survive. The conversation is related in a flashback, when the man remembers the woman's words relating her despair to him right before she apparently kills herself. The mother says,
"The one thing I can tell you is that you won't survive for yourself. I know because I would never have come this far. A person who had no one would be well advised to cobble together some passable ghost. Breathe it into being and coax it along with words of love. Offer it each phantom crumb and shield it from harm with your body".