Edna St. Vincent Millay may seem to have lived a looong time ago, but she actually lived in a world with plenty of electricity. So why would she use a candle as the central metaphor of her poem? Isn't that dated technology? Maybe Millay just had a thing for the good ol' days. But then again, maybe the candle functions as a perfect symbol: after all, Hamlet called life a "brief candle."
And if Shakespeare uses candles, then you know you're working in a pretty serious poetic tradition. Candles are common, which means everyone has some experience lighting one. Candles are natural (wax is from bees, remember?), so they're a fairly good substitute for natural human experience. And, with the advent of electricity, we tend to only use candles on special occasions—which makes them all the more striking in a poem.
You can't miss the reference to the candle in this poem. For one thing, it's the very first subject of the very first sentence in the poem. And if you missed it the first time around, the candle is also what every "it" in the rest of the poem (all three lines of it) refers to. We talk about it lots in the "Detailed Summary," but we'll give you a quick round-up here: candles have been used for, oh, a Very Long Time to represent human life—like since Shakespeare-time (remember how Othello snuffs out a candle before he snuffs out his wife?). And even before that, Bible time (when Matthew tells people not to hide their light under a bushel). You can read about both of those candles and their oh-so-interesting fates in Shmoopland, as well.
Candles have a natural end, like humans. And we often describe the time after death as a darkness, which is, well, sort of what happens when a candle goes out. So it's a natural go-to for any poet trying to discuss life (or death) in understandable terms.
Why does Millay pick such well-used imagery? Well, that's a more interesting question. There's something tongue-in-cheek about this entire poem, from its speaker to its meter to its symbols. Take a well-used symbol. Check. Then mess with it by making it do new things—like burn at both ends. It's gutsy, sure, but then, so is the poem itself.