Why does one eye become weaker than the other?

My left eye is weaker.

I’m sure my left brain is much larger than my right brain by now, but I am right handed so the muscles on the right side of my body probably get more exercise than that on the left. Are these even factors? It’s not like I walk around consciously using one eye more than the other, so there must be another explanation.

And is there anything you can do to help balance things out? Bad eyesight runs in my family, but I don’t want to make things worse.