I was always told that it’s done like that so it doesn’t get in the way of their dominant hand. The dominant hand (for most people this is the right hand) is involved in many more activities and therefore is at greater risk of damaging the watch. Plus it does get in the way when doing everyday things. If you are left handed you can always get a left handed watch (see related links)

It goes back to the time of wind up watches, the mechanism was easily accessible for right handed people to wind the watch.
I always wear my watch on the left hand because it would get annoying on the right hand. It makes things a little awkward when writing when you have a watch weighting you down, etc.
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i always thought it was because the knob to change the time is on the other side, and because when you have to change the time, if it was on your right hand, you would have to use your left hand.
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I am naturally left-handed, i wear my watch on the right because i don’t want it to restrict me when i write. Also it naturally feels at home better on my right hand rather than my left. men wear their wedding rings or watches on their left hand or arm