What are the causes mitral stenosis ?

Rheumaic fever: Usually repeated strep infections as a child lead to rheumatic fever which can scar the heart and make the valve thickened and hard to open.

Infections: Most common cause, although rare these days in the westernized world, is rheumatic fever (due to strep infectipns) in young age, which leads to scarring of the valve over time with narrowing - stenosis. Other infections (endocarditis) can lead to mitral stenosis as well.