My left hand is the one which has suffered the most the many hours of using a keyboard over the last +-25 years. While the right hand has the occasional break from the keyboard when using the mouse, the left hand is constantly glued to the keyboard.
It also has a much tougher job - all the cmd, ctrl, alt and shift + combinations are mostly done using the left hand - e.g. on Mac you cannot cmd+shift+ select text with the arrows - you must use the left hand - so it ends up doing so much more work.
I wonder if there are other people with the same problem. My right hand never hurts after many hours of computer work - but the left hand does. It hurts even now that I am typing and I haven't even spent more than an hour doing it.