It is because you are using your dominant hand. Try switching the mouse with your non-dominant hand.
For example as a kid I had to use left handed scissors or I couldn't cut things, now I use right handed scissors in either hand. I switch my mouse to my right and left hand sometimes without thinking about it. My brain just transparently maps the right click to my left hand index finger. I throw with my right arm and shoot right handed, but when it comes to tool use I seem to be ambidextrous. This said, I can only write with my left hand.