The arm you don't write with
The hand you don't write with.
If you're right handed you'll use the right hand/arm more than the left, which will increase the risk of the watch getting scratched. Also, right hande people are more prone to be carrying stuff in the right hand. Looking at the watch is easier if it's on the left hand that isn't carrying anything.
If you're right-handed, and write by hand, then having a watch on that wrist is quite awkward.
A Seiko automatic watch has less battery power than a Seiko kinetic watch. So if one purchases an automatic watch, they'd have to wear it daily. The kinetic watch has stronger battery power.
The arm you don't write with
Etiquette does not dictate which arm to wear a watch. This is mostly a matter of preference but most people find it comfortable to wear it on the left arm.
Left
Everyone that I know that is right handed, wears their watch on the left arm, regardless of their gender. Left handed people often wear their watch on the left arm.
men where watches on there left arm
The left wrist is the watch wrist for both men and women.
It doesn't matter; just preference!
Pilots us left arm for steering - right hand for controls - natural then for watch to be read on right arm.
men always on there left
Its really a simple reason. In combat you are less likely to damage the face of the watch when it is on the inside of your arm.
A women wears her watch on whichever side she feels like wearing it. In my experience, people tend to wear their watch on their less dominant arm. There are left-handed versions that have the controls on the left side of the watch case.
you will wear a cast or a brace