What would be the the cause of that the alternator keeps going bad in my car?

Often times an alternator failure is a result, not a cause, meaning there is something that caused its failure. Typical causes are a weak battery or one that will not allow a high enough voltage to be reached, poor electrical connections or wiring, or even a higher demand for electricity sometimes caused by add on components. All of these can overwork the alternator and cause it to fail. Any well trained mechanic with the proper test equipment will beable to test for these causes.