1998 Mazda Protege water pump replacement

How often should I replace my water pump, and what could either cause it to deteriorate prematurely or confuse a mechanic into thinking it needs to be replaced when it doesn't? I replaced my timing belt and water pump 2 years ago (4/06) and now my (new) mechanic tell me it needs replacing again. I brought it in last week because the engine was overheating. They replaced a temperature gauge and gasket but now say the water pump also needs replacement. I was ciofused, so I brought it back to my original mechanic who said there was no problem with the water pump that he could see, but that my oil pressure switch was broken. I have not put an excessive amount of mileage on my car in the past two years (approx. 20,000 miles), or driven it in extreme conditions. I really don't want to change the pump - and it goes without saying the timing belt too - after only two years, given the labor costs associated with this service. What is going on here? What questions should I be asking?
Thanks.