Because writing with my right hand is even more painful. Trust me, if writing with my right hand was a pleasant experience, I'd do it in a heartbeat. This isn't just some "I identify as a lefty; therefore, I do things left-handed" kind of thing. In fact, most other activities I do, I do right-handed.
Swing a bat/club? Right-handed. Throw a ball? Right-handed. Use a computer mouse? Right-handed. Shoot a rifle? Right-handed. Write a note? Left-handed. Paint a picture? Left-handed.
In the mid-90s at work, I was known as that guy who used a left-handed mouse at all times (I am right-handed.)
I would swap L/R button functions on the Sun4 3-button optical mouse and set it to the left of the keyboard. This is a comfortable and natural position, especially considering the placement of the numpad.
Very soon, the word was out in the office, and NOBODY attempted to borrow my workstation!
Swing a bat/club? Right-handed. Throw a ball? Right-handed. Use a computer mouse? Right-handed. Shoot a rifle? Right-handed. Write a note? Left-handed. Paint a picture? Left-handed.