A bad battery that is often low can cause not enough amperage to get to the starter to allow it to properly turn over which could damage the starter.
Water or mud also getting into the starter can cause a starter to fail prematurely too.
Mostly though it's just normal wear and tear of the starter when the bearings in the starter go out after turning over your engine so much.
A good starter on a vehicle should last from 5 to 10 years depending on how much you start your car.
Sometimes though it may not even be the starter that is bad and could just be the solenoid but on most newer cars it's just as easy and cheap to replace the starter and solenoid together since the solenoid is usually mounted on the starter anyway.