Why do you use your mouse with the right hand? I’m also left handed and use the mouse with my right hand, I think mostly because my parents bought a computer table with our first computer that had the spot for the mouse fixed on the right side (keyboard drawer with just enough space for the keyboard alone and below that a little extra drawer to the right to create space for the mousepad and mouse when the keyboard drawer is open). So I had to learn using the computer that way …
Seems inconsiderate from my parents, but I think they just didn’t think about that aspect.
Holding the mouse in the right hand allows for holding a pen in the left at the same time. I find that, and having the notepad next to the keyboard, to be quite useful. There's also no risk of the notepad being in the way of the mouse movements.
I also write left handed and mouse right handed. My theory is that it’s more efficient when using a QWERTY keyboard layout. The QWERTY layout is heavily left dominant in English and, I guess, many other Western European languages (https://en.wikipedia.org/wiki/QWERTY#Properties: “In the QWERTY layout many more words can be spelled using only the left hand than the right hand. In fact, thousands of English words can be spelled using only the left hand, while only a couple of hundred words can be typed using only the right hand (the three most frequent letters in the English language, ETA, are all typed with the left hand)”)
So chances are that the last key hit before using the mouse is on the left side of the keyboard, as is the first key you’ll hit after using the mouse.
That makes mousing with the right hand while keeping the left hand on the home row faster than the reverse.
This might seem odd for the youngsters having grown up with their own smartphone and laptop, but I very quickly found it annoying having to rearrange everybody's setup when using their computer. Friends, family, school, you name it...
I left mouse on work computers, althoigh I'm right handed, and right mouse on personal computers. (ergo at my first real job encouraged left mousing, and I found mixed mousing is best for my wrists)...
Anyway, most computers you find in the wild are setup for right mousing. Most mice are sculpted to be used by a right hand, a minority of mice are ambidextrous and can be used by either hand well (although it's not unusual for an ambidextrous mouse to have buttons for a right thumb and not for a left thumb), and I can't say I've ever seen one sculpted for a left hand. Same deal for trackballs; if you want a thumb trackball, it's got to be operated by your right thumb.
I'm left handed, use the mouse on the right, and even use my right hand for a laptop trackpad. I keep my left hand on the keyboard while mousing, and use modifiers in concert.
Perhaps this both explains why I've never quite understood the aversion to mice, and also why I prefer smaller (TKL or smaller) keyboards.
Seems inconsiderate from my parents, but I think they just didn’t think about that aspect.