FOOLISH ROMANTIC TEENAGERS are always thinking about love. They read about it, too. Look at an issue of Seventeen-- say, last month's--and you find a regular leitmotiv. There's an article called "Eight Love Traps," which tells the foolish teenager how to get out of difficult situations. Another, called "Inching Up to a Shy Person," explains how to get into a new one. There's a quiz-article called forthrightly "Are You In Love?"

Interest in these matters by no means ceases at 20. Glance now at the same month's issue of Cosmopolitan. Notice which articles are featured on the cover. One tells "How to Write the Perfect Love Letter." Another, promising considerably more drama, is called "When an Old Friend (Unexpectedly) Becomes a Lover." And for those who didn't learn back when they were teenagers, there is a piece called "What Is 'In Love'?"

Interest doesn't even expire at 65. Look at Modern Maturity, the magazine of the American Association of Retired Persons, for that same month. There's a lot here about hobbies and health. There's also an article called "Never Too Late to Date."

Most of this stuff is tripe. Tripe with a long heritage, to be sure. That piece on the perfect love letter harks back 21/2 centuries to Samuel Richardson, who invented the English novel more or less by accident while he was slapping together a manual on how to write good love (and good other) letters. It may even date back four centuries, to when Sir Philip Sidney was biting his pen and looking for the right words to impress young Penelope Devereaux. At the last minute he got help. The goddess Euterpe gave him what must be the shortest (and best) love-letter manual in existence.

"'Fool,' said my Muse to me, 'look in thy heart and write.'"

Sidney is not alone in avoiding tripe. There's even a fair amount of distinguished writing on the subject. There's Erich Fromm's The Art of Loving, for example--not that that focuses on romantic or sexual love. My favorite, though, is a book by the great French novelist Stendhal. On Love is a book-length meditation, written long before The Charterhouse of Parma or The Red and the Black. It has plenty to say about traps, shyness, old friends in new amorous moods, how to know when you're in love--though nothing about dating, which hadn't been invented yet.

Stendhal is pre-Freudian but psychological. He begins by recognizing that love is largely self-generated. The beloved is less a person one meets than a person one creates. This process Stendhal calls crystallization, taking his metaphor from the salt mines of Austria. He had seen miners there stick a bare twig into the saturated water and later pull it out covered with glittering salt-diamonds. Before you fall in love, you see the other person as a bare branch; as you fall in love, you coat him or her with jeweled attractions about 80 percent of your own making.

Next Stendhal turns to classifying the types of love. In his scheme, there are four main ones: passion-love (not what you think), sympathy-love, sensual love (what you think), and vanity-love. Of the four, only passion-love involves full crystallization, and only it truly wins Stendhal's respect.

Vanity-love is the lowest kind. Here you are out to please your own ego, whether through the delights of conquest or through loving someone who will impress your friends and make you envied. The person impresses you, too, of course. In Stendhal's own favorite quotation (from the Duchesse de Chaulnes in her later years), "A duchess is never more than 30 years old to a snob."

Sensual love comes next. It at least is honest and direct, being sheer physical attraction. But despite our own age's attraction to physical attraction, reverence for the orgasm, etc., Stendhal sees sensual love as fairly minor. Indeed, he claims that much of the time it's just another business transaction, "the only difference being that instead of money we earn pleasure."

Sympathy-love is rather better. This is love by the rules. You know what's expected of you; you have read the manuals, can write perfect love-letters, are passionate, casual, tender, playful, brutal, as the unwritten rules of your time and social group demand. Love is here a sport, the best one, the most complicated and the most serious.

Finally, towering above all these, there is passion-love. You can recognize it by several marks. First, it is never prudent. Not even one little corner of your mind is concerned with whether the person you love will wow your friends, or with how much pleasure you expect to "earn." (Often enough, none.) Second, it transcends self-consciousness. The person who imagines he or she feels passion-love yet can preserve dignity is, says Stendhal, "like a man who flings himself from a window and at the same time tries to reach the sidewalk in a graceful attitude." One more: In passion-love, sex is not the goal but a mere antechamber to something better still. As Stendhal says in one of his nicest phrases, in true passion "intimacy (he means in bed) is not so much perfect happiness as the last step toward it."

But the interest of On Love does not derive from this tidy classification scheme. It derives from Stendhal's countless asides, anecdotes, and minor insights. For example, have you ever known a married couple where one constantly picks on the other, and yet they stay together for 40 years? Stendhal knows exactly why--and analyzes it in the chapter called "Quarrelsome Love." Basically, he says don't feel too sorry for the one who is picked on; he or she gets to star in a life-long drama. "Where could one find, apart from passion-love, gambling, or the possession of power, any other source of daily interest to be compared with it (quarreling) in keenness? If the one who is always finding fault dies, the surviving victim is never consoled. This principle forms the bond of many a middle-class marriage; the person who is nagged at all day long listens to what interests him most."

Or, again, Stendhal has a lot to say about what he calls "failures" and we call male impotence. In fact, the chapter he devotes to failures might well be called "The Stendhal Report on Male Sexuality," and it is fascinating. His thesis is that impotence is not some kind of mechanical failure, but (for men) one more test of true love. "If a seed of passion enters into one's soul, there also enters a seed of a possible failure."

He has many stories to tell in support of this theory, such as that of the 23-year-old lieutenant of hussars who, "because he was too much in love, could do nothing but hug his mistress and weep for joy during the first three nights he spent with her." Or the frank conversation he once had with five handsome young Frenchmen. Except for one, who was almost certainly lying, "we all had failures the first time with our most remarkable mistresses." He even tells a story, which I shall not repeat here, about the unfortunate and super-romantic Englishman whom he calls "the king of failures."

That section of the book is every bit as relevant in 1981 as it was in 1822. I won't pretend that all sections are. After all, attitudes toward love and sex have undergone at least three major changes since Stendhal wrote. Much that he says is outdated. Furthermore, he wrote as a European to other Europeans, totally ignoring the United States, except to brush us off as a boring new little country which knew nothing of passion-love, being much too occupied with clearing land and going to church. Beyond that, he wrote as a man to other men; women are not addressed directly in this book.

All the same, if I were a foolish romantic teenager and reasonably literate, or just a person of any age interested in subtle reflections on love, I would step back from the magazine rack and hotfoot it to the nearest library. Or if I had $3.50, I'd pop into a bookstore and get the Penguin paperback version, which is called simply Love. The translation Penguin uses is not quite as good as the one I've been quoting (the very title Love slightly misrepresents De l'amour), but it's the only one in print, and it will do nicely. Like Tolstoy, Stendhal can survive any decent translation.