Why is clover with 4 leafs associated with luck?
Probably simply because they are rare. Perhaps the fact that we like symmetry helps. If something is both rare and pleasant to look at, we are likely to view it favourably and think of it as lucky. We then reinforce the idea that they are lucky through culture.
Like most cultural things it’s arbitrary, however I believe that the prevalence and longevity of this superstition is due to a couple of factors;
1.) They’re rare, but not so rare that you can’t find one if you look for it for a little bit.
2.) Clovers are all over the place, and so it isn’t very limited geographically, additionally they’re free, so the ‘lucky 4 leaf clover” wasn’t limited by class or geographically.
3.) It gives something for kids to do, and to get excited about. Which is invaluable as a parent, kids love rare, lucky things to search for, it makes any field a potential treasure hunt for children!
All of these factors combined makes the 4 leaf clover a perfect superstition to spread globally.
There isn’t a specific reason. But the idea dates back to the ancient Celtics. It’s probably because 4 leaf clovers are a semi rare mutation of normal 3 leaf ones.