what causes the water pump to fail?

[neanderthal]

id like to know exactly what it is that causes the waterpump failures that seem to be the bane of these cars.

i have a 1990 300E 2.6 which is leaking water from the region just behind the water pump and adjacent to the block. it gulps about half a gallon every other day and the temperature sometimes stays in the 100s even when on the freeway. oddly enough, if i turn on the heater it still blows hot, but previously when my windshield washer heating coil thingamjig was broken and my coolant was leaking into the washer bottle, this didnt happen. no heat, i guessed coz there wasnt enough water to pump to the heating system.
anyway, i dont know if the water pump problem is a failure of the actual part or a gasket or corrosion of materials or what.
is it the gasket deteriorating over time?
materials?
part of questionable build?
or just age?

and is there anything else that i should look at before condemning the water pump.
cheers