I noticed once I started having kids, I learned how to use my left hand for a lot of things. I don't have specifics here but it could range from opening something with your left hand because your right was already full.
left handed, broke that hand and learned to write with my right hand, never had left handed scissors in school so learned with right ones. I kick with my right foot(dont know why?), throw left handed, bat left handed, golf left, in school i always put the mouse on the left side but it became a pain to keep switching so i turn to using on the right side.
When I was little, left handed scissors were hard to find, so I learned to cut with my right (now I can do both). I also use a knife right-handed. The computer mouse I can use either way, but like Chilipepper said, it's a definite advantage being able to mouse right and write left