I so so love you. With five unlikely little words, uttered within seconds of being elected, the new leader of the Labour Party showed the world that he was a sympathetic kind of guy who was prepared to let his feelings show, whether he felt them or not. Not so long ago, this kind of declaration – particularly to a brother – would have set alarm bells ringing but, in 21st-century, loved-up Britain, we are altogether more sophisticated about the world of the heart. We know where emotional exhibitionism ends and true feeling begins. We have learned the "10 Golden Rules of Modern Love".

1. The more someone declares love for an individual from a public platform, the less it means. Invariably, the "I love you" is an expression of some kind of guilt or is a marketing exercise. The fraternal tribute of Ed Miliband was as genuine as Bill Clinton declaring his love for Hillary during Monicagate. The hope in both cases is that a massive love bomb of nuclear proportions will obliterate any trace of bad behaviour. The secondary subtext to these pronouncements is often to remind the audience that the speaker has a warm and giving heart. Only rarely (Gordon Brown's references to his wife Sarah, for example) do they ring true.

2. What grammarians call "a repeat-intensifier" – for example, "I so so love you" or "I really really really love you" – has the paradoxical effect of lessening the sentence's sincerity. In terms of sense and rhythm, there is nothing that anyone can do to make the most famous three-word sentence in the English language more heartfelt or convincing than it already is. "I love you" can never be more than "I love you". Those who try to extend it through repetition are often trying to cover up a lack of true feeling.

3. Public displays of love are always suspect. When Al and Tipper Gore snogged on a public platform, divorce lawyers started licking their lips. It was when Tom Cruise leapt up and down on Oprah Winfrey's sofa, declaring his adoration of Katie Holmes, that world experts on love began questioning their marriage.

4. For the same reason, it is always a bad sign when someone expresses lifelong love in the form of a tattoo, however beautifully drawn. When he fell in love with Katie Price, Peter Andre told the world the words "I love Katie" had been emblazoned on one finger. The tattoo has recently been removed.

5. The words "I love you" said before or during sex are meaningless – or, rather, they mean something entirely different from what they actually say. When uttered in a post-coital moment, they are marginally more significant, although they are quite often used when a simple "thank you very much" would have sufficed.

6. It is the declarations of love, delivered without a drum-roll, which matter. The relatively new habit of ending telephone conversations with "lots of love" or, when appropriate, "love you" has made the world a slightly friendlier place, expressing an easy, everyday affection which may well matter more than a great romance.

7. When someone expresses his or her love for a wife or lover while surrounded by other people, something faintly whiffy – a complicated domestic game – is afoot. It is bad form and should be slapped down by other guests.

8. When family members start declaring their deep affection for one another in front of outsiders, an act of unsubtle boastfulness is taking place.

9. When a husband or wife starts repeating the words "I love you" with the regularity and sincerity of a cuckoo clock, he or she is almost certainly having an affair. Since the dawn of romance, the confusion between expressing love and feeling it has been exploited by those behaving badly.

10. Sometimes, particularly in these islands, the surest and most reliable way of expressing love is by keeping quiet, by using a look or a gesture, and leaving those increasingly shop-soiled words "I love you" to politicians and celebrities.