FBinNY 

Cables fatigue fray and break. it's normal, though their life expectancy varies tremendously. The cause is the winding and unwinding of the arc of the lever. It's like bending a paper clip back and forth, after a while it stiffens, gets brittle and finally snaps.

Note, that it's the bending action, not the tension that causes the problem, and the life expectancy depends not on how many miles you ride, but how often you shift. Folks riding in places like Pennsylvania, and Connecticut riding rolling terrain can expect much shorter life than folks in Kansas or even Colorado, where shifting is less frequent.

BTW- an interesting note is that modern die drawn wires such as used on the best systems are more prone to flex fatigue than the less expensive stuff. But all things considered, gear cable replacement is one of the lowest per mile maintenance costs, even in Connecticut.