1. Less damage: You tend to use your dominant hand more and are more active with it, it’s easier to accidentally scratch or break your watch when wearing it on your right hand. This is the key reason why left hand watch wearing is the norm.
2. Easier to set: Majority of watches have their crown on the right side of the watch case. If you are wearing your watch on the left hand, the crown becomes easily accessible, making it easy to change the time. If you, on the other hand, wear your watch on the right hand, it becomes essentially impossible to change the time with your watch on your arm. Which means you would have to remove the watch in order to set the time.
3. Easier to check: if you are righthanded and you wear your watch on your right hand, your watch might get in the way, as well as making it hard to twist your wrist to check the time. As such, your watch sit uncomfortably as you’re performing tasks using your right hand.
Conclusion: Don't get me wrong, wearing wrist watch on either your right hand or left hand are acceptable but these are the reasons why left hand is more preferable than the right hand.
Generally gents wear in left hand and women wear on right.