The classic belief about four leaf clovers is that they bring good luck. My informant doesn’t remember where he heard it except that it seemed to be common knowledge from an early age. He never believed it, but when he was fifteen, he found three in one day, later framing them to remember it. The next day, though, he won a martial arts tournament, beating a few people he consider to be more skilled than he was. After that, he told me he always kind of believed that four leaf clovers could bring good luck, even if there was no physical reason they should.
I’ve heard that Eve was said to have taken a four leaf clover with her after she was exiled from the Garden of Eden. That’s said to be a reason they bring luck. I’ve also heard that the four leaves represent faith, hope, and love, and luck. The best reason I can think of for why people associate them with luck is simply their rarity. I’ve been told they’re only supposed to occur once for every ten thousand clovers. Humans often attribute some kind of larger significant to events just because they’re rarer, so it makes sense that we think four leaf clovers are lucky. It is a way of trying to find order in a chaotic world basically. There are billions of people in the world and most of those people will travel to many different places over the course of their lives, lives which last many years. So it makes sense that some people will run into four leaf clovers at some point, but humans still think it’s a very special thing to find one. Therefore, we say they bring good luck.