What causes 'bad sectors' on a hard drive?

Hello there

Thanks for the links (which I have read). Unfortunately they did not tell me anything I didn’t already know; what I would like to understand is the cause of bad sectors on MY hard drives. I have had two drives fail in quick succession (and a third in the process of failing) WAY sooner than they should have. I want to understand the reasons why - it is possible *I’M* doing something wrong (however, I think this is VERY unlikely!), and I want to correct this ASAP if this is the case. If it’s not my fault, then I want to know why the drives are failing before their time.

But thankyou anyway.