Why do we light candles on top of a birthday cake when celebrating someone’s birthday?

Because it is too hard to put them on the bottom of a birthday cake (bah dum dum). Actually this custom can be traced back to ancient Greece when the moon goddess, Artemis, used to make offerings of cakes with circles of candles representing the full moon. Several centuries later, the Germans put a single candle on a cake to represent the spirit of life.