Because loving is reciprocal physiologic influence, it entails a deeper and more literal connection than most realize. Limbic regulation affords lovers the ability to modulate each other’s emotions, neurophysiology, hormonal status, immune function, sleep rhythms, and stability. If one leaves on a trip, the other may suffer insomnia, a delayed menstrual cycle, a cold that would have been fought off in the fortified state of togetherness. (208)

People differ in their proficiency at tracing the outlines of another self, and thus their ability to love also varies. (207)

In all of its varied and protean forms, love is the tether binding our whirling lives. Without that biological anchor, all of us are flung outward, singly into the encroaching dark. (225)

Loving is limbically distinct from in love. Loving is mutuality ; loving is synchronous attunement and modulation. As such, adult love depends critically upon knowing the other. In love demands only the brief acquaintance necessary to establish an emotional genre but does not demand that the book of the beloved’s soul be perused from preface to epilogue. Loving derives from intimacy, the prolonged and detailed surveillance of a foreign soul. (207)

Mutuality has tumbled into undeserved obscurity by the primacy our society places on the art of the deal. The prevailing myth reaching most contemporary ears is this: relationships are 50-50. When one person does a nice thing for the other, he is entitled to an equally pleasing benefit – the sooner, the better, under the terms of this erroneous dictum. The physiology of love is no barter. (208)

Modern amorists are sometimes taken aback at the prospect of investing in a relationship with no guarantee of reward. It is precisely that absence, however, that separates gift from shrewdness. Love cannot be extracted, commanded, demanded, or wheedled. It can only be given. (208)

When people have trouble with their emotions – a bout of anxiety or depression, say, or seasonal gloominess - they often want science to pinpoint an offending neurotransmitter in the way that a witness picks the perp out of a lineup. Is it excessive norepinephrine, too little dopamine, errant estrogen? The answer is apt to dissatisfy: no single suspect can be fingered with confidence because the question itself attributes a fallacious simplicity to the brain.(91)

One brain’s blueprint may promote joy more readily than most; in another, pessimism reigns. Whether happiness infuses or eludes a person depends, in part, on the DNA he has chanced to receive. (152)

When trying to fathom an immense, intricate system, drawing direct arrows of causality between micro and macro-components is perilous. Which stock caused the crash of ’29? Which person triggered the outbreak of World War I? Which word of Poe’s “The Rave” suffuses it with an atmosphere of brooding melancholy? (91)