When top London psychotherapist Frank Tallis was consulted by a stalker suffering from an extreme case of unrequited love, he was struck by how balanced the woman seemed in other aspects of her life.

The 40-something worked as a lawyer’s clerk and was happily married, but had fallen head-over-heels for her dentist, who’d exhibited a nice bedside manner after a particularly nasty surgery.

Her feelings weren’t reciprocated in the slightest and the dentist fled the country to get away from his psychotic patient.

“Pathological love is much stronger than ordinary love,” Tallis tells The Post. “Megan’s experience [all patient names have been changed] demonstrates how we can feel mentally safe and secure when we are, in fact, walking on a precipice and can tumble over at any time.

“The line between normality and abnormality is blurred by love.”

It’s a theme that anchors Tallis’ upcoming book, “The Incurable Romantic: And Other Tales of Madness and Desire” (Basic Books, out Tuesday). Part memoir, part scientific exploration, the collection chronicles more than a dozen tales of love gone horribly wrong.

As Tallis explains, there is a spectrum of love-fueled insanity — ranging from a patient who developed a narcissistic yearning for his own body, preferring masturbation to his own partner, to an elderly widow who suffered from hallucinations because she missed the rampant sex she enjoyed with her husband, with whom she had nothing else in common.

Now retired, he has drawn on his 20-year experience as a clinical psychologist to write the remarkable work of nonfiction. After researching his 2004 book “Love Sick,” Tallis found himself increasingly drawn to the darker corners of the amorous mind.

Megan, the jilted stalker, is one of his more bizarre cases. She suffered from the rare de Clérambault’s syndrome — named for the French psychiatrist who first detailed the diagnosis in 1921 and classified less lyrically in modern psychology as “Delusional Disorder: Erotomanic Type.”

“It is effectively a delusion of love,” says Tallis. “The individual is usually a woman who falls for a higher-status man who has given no indication of interest whatsoever.

“She believes that the passion is reciprocated and, even if the target is avoidant, it’s because they are overwhelmed or frightened [by what she perceives is the truth]. The syndrome patient usually tries to persuade them that they are in some kind of spiritual union.”

Megan bombarded her dentist with calls while stalking him at his home and office. She made his life such hell that, after many months, he eventually emigrated from London to Dubai, United Arab Emirates, with his family. Megan was left agonized, although her husband continued to stand by her.

“She had been enjoying a perfect life, which was turned upside down by this absolute and inexplicable passion,” he says.

As for the cause, nobody can know exactly why the clerk — who did not respond to antipsychotic medication and had no understanding of why the dentist had fled — behaved in this way.

“There is no clear explanation why de Clérambault happens,” Tallis says.

Another form of obsessive love centers on the case of a wealthy, married man called Ali. He went bankrupt after spending millions on no fewer than 3,000 high-class escorts.

“At first, I thought he was a sex addict, but it turned out he was addicted to the feeling of having women fall in love with him,” says Tallis. “One could scientifically speculate that the chemical changes he experienced in his brain were the source, but there were probably a lot of other factors involved.”

Ali spent so much money on the prostitutes because he desperately wanted them to fall in love with him. He would shower them with gifts and even take them to look at fabulous houses with a real-estate agent.

“He would promise them a better life. Then, at the point where they said, ‘I love you,’ he would drop them and proceed with someone else,” says Tallis.

Frustratingly, Ali stopped coming to his appointments after the fourth meeting when he finally disclosed his pricey indiscretions to the shrink.

“For a lot of people, therapy is about confession,” says Tallis, who, to his chagrin, never received payment for any of Ali’s sessions. “Somehow it is magically unburdening and they feel they can carry on.”

But what about less dramatic examples of disordered love? Tallis says we can all fall victim to it, especially in the age of Facebook when we can easily keep tabs on — or “stalk,” as he puts it — ex-lovers. Technology has helped facilitate people’s propensity for obsessive love.

“Evolutionarily speaking, the person you had sex with is extremely important for the continuation of the species,” he says. “In a sense, our ex-lovers are part of the construction of our personhood. When we Google them, we aren’t just looking for them, we are looking for ourselves. It’s a very powerful thing.”

Meanwhile, married for more than two decades, Tallis, who also writes fictional thrillers, insists that his research into the subject has not damaged his own take on love.

“It makes you a lot more grounded if you know how easy high romance can take you away from reality,” he says. “I’m more pragmatic in relationships, but I don’t think it’s marred the enjoyment of romance.”

As for a cure for “Crazy, Stupid, Love,” Tallis is loathe to explain that there probably isn’t one.

“My book’s called ‘The Incurable Romantic’ for a reason,” he says. “When my wife read the manuscript, she asked, ‘Do you really want to write this because people will just think you’re a really bad therapist?’

“But I’m trying to be honest about the process. To underscore that the conditions of longing and desire — the pathologies of love — are really profoundly deep, difficult to treat and they should be taken seriously.”