Roughly speaking, about 10% of the population are left handed. Nobody knows exactly why some people are right handed and others are left handed - but it's interesting to note that chimpanzees display left and right handed characteristics in similar proportions to humans.
Right handed people wear their watches on their left wrist in order to be able to tell the time without stopping what they're doing with their "main" hand. For example, telling the time whilst writing, drinking a cup of coffee, or just flicking through the channels on the TV is a lot easier when you're watch is on your "secondary" wrist.
Off course, lefties can just wear a "standard" watch on their right hand - but then the controls are on the wrong side of the watch for easy access using their left hand.
The solution is to get a left handed watch, one which has the controls in the correct position for wearing on the right wrist. Fortunately, there are plenty of different left handed watches available - and these don't cost anymore than standard right handed watches.
Listed below are a few examples of Invicta left handed watches for you to have a look at. Invicta has a really wide range of watches for lefties. There are lots of different colors and styles available.