Candles
A candle loses nothing when it's used to light another candle.
A candle gives a soft, warm light and there doesn't seem to be much else to say about it. Other than because a candle gives a soft, warm light, it's usually considered a 'good' thing. It is aesthetically more pleasing to have a candle-lit dinner than to dine under an electric lightbulb, even though the latter gives more light to see what you're eating.
Candles are more than just lovely ornaments; they add a bright and warm ambience to any situation. Whether scented or plain, or even packed with dust to emit sparks when lit, candles feel alive.
Candles

Le jeu ne vaut pas la chandelle is what the French might say about English 'cuisine'. It means 'not worth a candle', and implies English food is so lacking in appeal that it's not worth using a candle to create sufficient light to see it.
'Not worth a candle' can be used to deride anything; an object, person or activity, which we want to say is worthless. Even though a candle is so cheap, using it would nevertheless be a waste of money.
Yet even though candles are cheap, we use them on very special occasions. Take, for example....
Weddings - unity candles
During some wedding ceremonies, the mothers (or both parents) of the bride and groom reverently light two small candles, which the bride and groom then use to light a single larger "unity" candle.
Carrying the flame from their parents' lights and merging them into a single flame, symbolizes the joining of the two families (Gen. 2:24). The lighting also invokes the Light of Christ, which is hoped will guide the couple through their married life together (John 8:12).
In contrast to the bride's veil and wedding rings, the so-called Unity Candles are not an ancient Pagan or Christian rite; rather a relatively modern idea, growing popular since the 1970s.
That said, the Pastor's Handbook of the Free Methodist Church (USA) says "The custom of having the bride and groom light a unity candle to symbolize their union in Christ is based on an ancient Christian tradition." but gives no details of this "ancient Christian tradition". If you can shed light on this, please let us know!
Whether or not it's an ancient tradition, since the dawn of time the flame must have been a mysterious phenomenon. When first discovered, humans must have wondered why it was so hot, where it came from, why it spreads, and where it goes when it disappears.
It's potential harm must have been quickly feared, yet what a life-changing event when a flame's value was discovered and harnessed for heating and light.
People who developed an esoteric insight into such mysteries have been called mystics, and a relatively recent one was Baal Shem Tov (1698-1760). He taught the direct connection between every human and God, the importance of prayer, and that we should follow our wandering thoughts to their roots in the divine. He is credited with saying:
״מכל אדם ואדם עולה אור המגיע ישירות לשמים. וכאשר שתי נשמות שנועדו להיות יחדיו מוצאות זו את זו, קרני אור זורמות יחדיו וקרן אור אחת בהירה יותר יוצאת מהווייתן המאוחדת״
“From every human being there rises a light that reaches straight to heaven, and when two souls that are destined to be together find each other, the streams of light flow together and a single brighter light goes forth from that united being.”

At about 13 lumens of visible light and about 40 watts of heat output, a candle is well-suited for representing married life. It is ignited by the heat of another flame, just as the bride and groom's wedding is celebrated by their families and friends.
As the candle wick is lit, it first melts and then vaporises a small amount of the candle wax fuel. Once vaporised, the fuel combines with oxygen in the atmosphere to form a flame. This flame then provides enough heat to keep the candle burning via a self-sustaining chain of events: the heat of the flame melts the wax at the top, the liquefied fuel then moves upward through the wick by capillary action, and the liquefied fuel is then vaporized to burn within the flame.
Once a couple begin married life together, they can enjoy sustaining each other through the rest of their lives.
A: They only work on wick-ends!
A unity candle can be stored for a lifetime, and lit during wedding anniversary dinners. So glowmantic!
Some churches encourage couples to have their unity candle ceremony during the reception, to avoid taking focus away from the central ritual in the wedding ceremony; i.e., the exchange of vows.
A popular candle arrangement at the entrance of wedding receptions is a dozen or so small candles arranged in the shape of a heart. The heart has long been a symbol of love, and a candle heart layout gives a special warmth.
When did candlemania start?
Candles didn't really reach the illumination of (olive) oil lamps in the Middle East until the 5th century, but the ritual and symbolic significance in pagan religions existed by around the 1st century across the Roman Empire and beyond. This was to honour gods such as Vesta, Apollo and Isis. The Greeks similarly honoured Hestia (goddess of the hearth) with a perpetual flame.
The presence of fire was often seen as a sign of divine presence. The light from candles were offerings to deities to signify devotion. Light was also believed to purify and protect sacred spaces from evil spirits. Lighted candles were placed in a tomb during a funeral, to help to guide the soul to the realm of the dead.
Christians liturgy started to include candles around the 4th century, especially on altars and during baptisms. By the Early Middle Ages, candles had symbolic use during the reading of the Gospel - the flame representing the light of Christ, purity, prayer and reverence. Specific rubrics for candle use developed for Mass, processions and "Exposition of the Blessed Sacrament".
Sources and further reading:
- "Lamps and Wax Candles"
thejournal.org/studylibrary/two-babylons/lamps-and-wax-candles.html - "Lamps and Lighting, Early Christian"
encyclopedia.com/religion/encyclopedias-almanacs-transcripts-and-maps/lamps-and-lighting-early-christian - "Why do we light candles?"
omhksea.org/archives/1052 - "The History of Votive Candles"
catholiceducation.org/en/culture/the-history-of-votive-candles.html - "Light of candles shines in history"
evangelist.org/news/2018/apr/06/light-of-candles-shines-in-history