Identical parts have a 'mean time between failure' MTBF rate. Have you noticed how one headlight or taillight goes out and its twin follows? Wheel bearings are pretty much the same. If one side goes out, its best to replace the other side at the same time. U-joints are the same, if the front goes, the rear isn't far behind. As far as wheel bearings, look for made in USA. There are a lot of cheap, Chinese bearings on the market. Some unscrupulous shops will cut corners and buy the cheaper part to increase their margin.