I have at home a box of old teeth. These are arranged by colour upon a wheel: unnaturally white through familiar cream, through the tobacco stains of yellow and brown to the foulest greens and grey that are hard to imagine in the mouth of any living creature. They constitute a dentist's guide to colour-matching, and accompanying the grisly colour-wheel are some small medical bottles, hoses and other whatnots.

The box dates back to the 1950s and was given to me as a Christmas gift shortly after its original owner died. As I have no practical need for dental tint-comparisons, the wooden case sits on display among various oddities that I have collected since being a student: taxidermy, pickled animals and amusing quack remedies line my shelves as an effective deterrent against any unwanted sexual attention.

Having known the dentist a little, and having a real fondness for his memory, it sometimes feels like a shame to me that this box, which was not long ago brought out every day by a noble and dedicated man to fit his patients with synthetic teeth in order to improve their lives and self-respect, regardless of the malodorous depths of dental hygiene to which they had neglectfully plunged, is now a mere droll oddity on a shelf. It is displayed for purely aesthetic value, its new owner ignorant of the precise use of the numerous pipes and pipettes. It has, in a sense, lost its bite. Such collections of unusual surgical and natural history paraphernalia can tread a fine line between fascinating oddness and cheap schlock. I worry sometimes that I am belittling a man's career to the status of a dusty amusement that I can point out to politely fascinated dinner guests.

Likewise, there hangs in the air something anaemically dissatisfying when people talk about Christmas as being "a time for giving", or "a time to remember those less fortunate than ourselves", or any other of the bloodless blandnesses that are vapidly trotted out around this time of year by priests and politicians. Does that not sap the true meaning of Christmas? Are we not missing out on its magnificence and reducing it to the status of a relic on a shelf that we don't quite know what to do with? Well, perhaps, but there are plenty of historical artefacts better left on quaint display than put to their original uses.

The diluted secular dicta do sound as though they lack force. But the dissatisfying insipidity of these worldly edicts comes from the obviously begged question: should we not give generously and think charitably at all times? And are we at all likely, during the hectic shopping sprees and binge-drinking more commonly associated with the yuletide season, to remember to be nicer to anyone?

We do not, despite the smug assurances of many believers, ultimately get our morality from the Good Book. The Bible contains so many directives and prescriptions for behaviour that range from the beautiful to the rapacious and repugnant; therefore, as Richard Dawkins elegantly points out, we must call upon a different means of deciding what constitutes honour, virtue and integrity, to cherry-pick from its pages the ethical advice we feel we should apply today. That intuitive source must lie elsewhere: it arises societally, and speaks of our nature as co-habiting human beings. To think of being kind at Christmas, then, is not simply to pay lip-service to a watered-down version of what the season "really" means, but rather to remind ourselves of our potential to shine as human beings.

The Victorians spoke much about "open-heartedness" and "benevolence", and there lingers a tweeness associated with the notion to this day; conversely, Freud's later legacy has us worried that any act of kindness must come from a selfish or even an aggressive place, where we act kindly only to be loved in return, or to manipulate or control. Meanwhile, we can easily think of flaccid, perpetually exploited people who relentlessly give of themselves to their own detriment, and give compassion a bad name. All told, kindness is not fashionable. We are told by lifestyle gurus that we cannot live productively without Setting Goals and learning How to Get What We Want, as if the key to life is to single-mindedly turn every situation to our own advantage. That's a revolting mantra, and it misses what makes us successful in so many ways, as well as happiest and most loved.

Successful, because kindness breeds kindness: this latter maxim is part of the bible of persuasion tactics. Do someone a favour and they'll feel obliged to reciprocate. If you want something from somebody, be sure to give them something first. Happiest, because acting kindly simply makes us feel happy. New cars and houses make us happier for short, bright bursts before we revert to our default level of contentment, and their erstwhile pleasures soon slip by unnoticed in the same way that we quickly stop hearing the sound of air-conditioning in a room. We do not so easily adjust, however, to the pleasure taken from acting altruistically: when we do kind things, we feel good and we continue to do so; our happiness level is raised and that default is set higher. Being nicer makes us happier.

Above all, kindness is that quality that we most like in other people. We try to be clever and witty around clever and witty people, forgetting that we don't especially like clever and witty people ourselves unless they are also delightful and charming to be with. Attempting needlessly to look and sound like those whom we want to like us, we ignore the fact that we don't especially like people who share our tastes, unless they also have that quality of loveliness that sets them apart. Worrying greatly about how we should best present ourselves to others, we relentlessly misjudge and try too hard. To simply be generous, open and engaged, on the other hand, is a simple recipe to appear likeable to anyone.

Most people think themselves kind enough, but rather like a magician thinking he is fooling an audience who can see through his tricks, we are the worst judges of the effect we have on others. True, we can mentally point out various kindnesses we have committed and those pleasant aspects of ourselves. Yet by doing so, we ignore the real test cases: how we behave under pressure; how nice we are to people we don't like; how we deal with other people who seem determined not to live up to our unrealistic expectations. I try to be kind where I can, but I fume and bubble when people let me down, as if they had nothing else to do but to pander fussily to my whimsies. Plato is credited with the saying: "Be kind, for everyone is fighting a great battle."

To talk secularly of Christmas being a time to remember others, then, does rather anaemically miss the point, but it is certainly as good a time as any other to rise to the challenge of leading a kinder, lovelier life – one that stretches far beyond the encouraged sentimentality of the holiday period.

As ever, the journey is the thing, and should be enjoyed accordingly. To forgive purely because it is nicer to forgive, and to do so when it's a tough call; to try to speak only kindly of those we know because it is preferable to do so; to enjoy the successes of others because living thus is more enjoyable than the stress of living resentfully: such kind things make us better, lovelier people. And to try to live this way for its own merits, without invoking a supernatural reason for doing so, is to celebrate our humanity and to give kindness back its teeth.

• This is an abridged extract from The Atheist's Guide to Christmas, edited by Ariane Sherine