Lisa Appignanesi is an ambitious writer who is temperamentally drawn to big subjects. Her previous book, Mad, Bad and Sad, explored the disturbing history of the plight of women whose mental state did not conform to contemporary notions of "health". Her memoir, Losing the Dead, was a compelling account of her own family history, in which loss, memory and desire played haunting roles. Now, in All About Love, she has turned her attention to the subject where most of us locate our beginnings, which none of us escape and which at some point in our lives will generally threaten to take us over, like a madness.

Appignanesi is refreshingly candid about what she is not going to be dealing with. She elects to concentrate on the culture of the western world, while acknowledging the influence of the east; she chooses not to "single out homosexuality", although she includes homosexual accounts of loving in her research; and she avoids the more sensational reaches of "love" – sadism, masochism and necrophilia – feeling that living in times where excess is "rampant in the media", a rebalancing focus on "ordinary love" is extraordinary enough. But this, of course, poses the really vexed question. What on earth is "ordinary love"?

Feste, one of Shakespeare's shrewd fools, offers a characteristically misty answer to this question. "Tis not hereafter…" he sings, by which he means not just that love is of the moment, but that it is easier to suggest what love is not than what it is. It is not, for instance, as Appignanesi points out, a matter of changes to the neural synapses or the lighting up of critical areas in the brain. That all emotional states have a physical corollary is not news; we are psychosomatic creatures whose minds and bodies are in cahoots.

What emerges from the book's attempt to chart the "arc of love" is that, like time, God and happiness, this is a subject resistant to analysis, because its existence is predicated on experience. It is possible, sometimes, to say what time, belief in God or being in love do to us; but this gives no real clue to what these words stand for. "What is your substance, whereof are you made?" as Shakespeare asks elsewhere, and does not stay for an answer.

Nonetheless, Appignanesi has a stab at providing an answer. Her account – which moves from first love to married love, to adultery, families and friendship – is enlivened with personal anecdotes; this is the yeast that leavens the dough of information. Her references are wide-ranging and eclectic – from the ancients to Oprah – mirroring the mish-mash of attitudes that love provokes. Significantly, much of her material derives from poets and novelists, on the basis that reality is often more accurately conveyed in fiction than via reported "fact", whether scientific data or personal record. What we "make up" as fiction is less likely to be tainted by the usual human defences against self-revelation and self-knowledge. As Oscar Wilde knew, "The truest fiction is the most feigning".

Appignanesi is particularly strong on the Russians: Turgenev and Tolstoy are key witnesses on the delusion of "first" love (she quotes from Turgenev's deeply intelligent story "First Love": "I had no first love... I began with the second") and the emotional economy of love triangles. Appignanesi has co-authored a book on Freud, and the influence of psychoanalysis underwrites her account of both these states. The ecstasy of first love recapitulates the oceanic feelings of the infant at the breast, and its perennial anguish derives from the anticipation of an inevitable second banishment, which mirrors the childhood expulsion from that Eden. The adulterous triangle is, unsurprisingly, the natural offspring of the Oedipal triad. But she also gives a valuable account of the role of love in the development of Freud's theory of analysis, which was, at heart, Romantic. Freud was much impressed by Wilhelm Jensen's novella Gravdia, from which he derived his idea of love as a catalyst of self-knowledge capable of unblocking psychosexual resistance. Appignanesi quotes Freud: "Every psychoanalytic treatment is an attempt at liberating repressed love which has found a meagre outlet in the compromise of a symptom." More urgently he wrote: "In the last resort we must begin to love in order not to fall ill, and we are bound to fall ill if… we are unable to love."

Understandably, much of the book is about love's shady cousin, sex. For one thing, sex is more tangible as a subject, more amenable to documentation and statistics, and therefore, one senses, Appignanesi falls back upon the sexual with some relief. But the conclusions she draws are somewhat depressing. By her account, the split between sex and emotion in modern youth, fuelled by increased drug use, is growing wider. On the other hand (or, more accurately, by the same token), there is the new enthusiasm for the cult of celibacy. I'm sceptical about these so-called "modern" phenomena. Stendhal, whom Appignanesi quotes, gives a pretty clear-headed account of that psyche/sexual split, as does Tolstoy, who, after a life of erotic abandonment, in his later years fanatically cultivated celibacy.

Perhaps because, as she herself has acknowledged, love of her children is the love story Appignanesi is now most stirred by, the most successful section of the book is on love in families. She quotes the Earl of Rochester – "Before I got married I had six theories about bringing up children; now I have six children and no theorie" – and something of the earl's agreeable pragmatism informs her thought. She is tender, without being sentimental, about the passionate attachments babies evoke, as well as need. But she is also good on hate, that vital feature of family life often brushed under the carpet. (Freud never discussed the fact that it is Jocasta's attempt to murder her son that leads to his incestuous union with her.)

Friendship completes the arc of the book and Appignanesi again comments on the contemporary ethos that makes her want to conclude on a temperate note. "It is unromantic civility and quotidian generosity that encourage our intimacies to endure." Undeniably true. But the life of the book lies is its author's apprehension that temperance is, alas, not what we crave. Love, for all its pains – or because of its pains – offers a promise of meaning, no matter that the nature of this "meaning" is forever deferred. Appignanesi quotes the psychoanalyst Adam Phillips's observation that our desire is always in excess of the object's capacity to satisfy it. There is a sense in which the worldly wise Feste is wrong: love is "hereafter", because its meaning lies precisely in the excitement of a promise that never loses its tarnish by being fulfilled.

Salley Vickers's collection of stories, Aphrodite's Hat, is published by Fourth Estate