Whether "love" should be the reinforcer is a question--and anyway what is "love"? Attention, smiles, encouraging verbal behavior... Brooks and others need to define it in observable and measurable terms (not easy for "love"), if they claim it's the conditioning stimulus (i.e., reinforcer).
So what about unconditional love? It teaches a child that whatever he or she does is OK--or does it?
Right! thinks the child. It doesn't matter what the child thinks in the moment. That's the part a lot of modern American parents don't get. They see only the short view (am I popular with my child? Does my child love me?), not the long view (Is my child turning out independent, knowing right from wrong, interested in the world, and able to get along with others?).
And there you have my opinion of the four worthwhile goals parents have for their kids.