As is often the case, it all goes back to Eve. Legend has it that as she and Adam were being hustled out of the Garden of Eden, she plucked a four-leaf clover to carry with her as a souvenir of Paradise [source: Elmerhebe].
The druids of the ancient Celtic world wouldn't have known Eve from Adam, as it were, but they too were big fans of four-leaf clovers and carried them around to ward off malevolent spirits [source: Ventura]. This practice evolved into a medieval theory that a four-leaf clover would give you the ability to spot fairies and take evasive action if necessary. As a result, back in the Middle Ages, kids entertained themselves by ferreting out the necessary stems and heading out for a fairy-hunt. Obviously, that's a video game just waiting to happen! [source: Evans-Wentz]
With the little green plants enjoying such popularity, it's no wonder St. Patrick decided to use them as a teaching tool when he set about converting Ireland to Christianity. The four-leaf variety being in short supply, he settled on the ubiquitous three-leaf clovers to explain the three-in-one nature of the Holy Trinity to the heathens. One leaf stood for the Father, one for the Son and the third for the Holy Spirit, all united on the single stem of the Godhead.
Similarly, a poem in the popular tradition holds that the four leaves on the lucky clover signify fame, wealth, health and faithful love. Along these lines, the English have a tradition that if you dream of clover, you're guaranteed a happy and prosperous marriage [source: Faust]. West of England, in Cornwall, some people alleged that if pixies stole your child and left a changeling in its place, the only way to get your own offspring back was to lay a four-leaf clover on the impostor [source: Evans-Wentz].
So we've established that four-leaf clovers have been considered powerfully lucky for a very long time, but why?