Why a secret keeper?

[pippin_999]

No: HPFGUIDX 189897

Wendy:
> Forgive me if this has already been discussed.  Ever since I read
> about secret keepers I wondered why have them at all?  Why do you
> have to tell your secret to someone?  Why couldn't Lily and James
> kept the secret to themselves?  It seems safer.  What is it about
> Wizards that requires them to have a secret keeper?
> 
Pippin:
Hi, Wendy and  thanks for de-lurking! 

JKR explained in an interview that if James or Lily had been the secret keeper, Voldemort could have   tortured or tricked or enchanted that one  into betraying the others. This way, even if Voldemort captured James or Lily, the other Potters would remain safe. 

So I guess the answer to your question is, wizards understand that even strong-willed people are vulnerable to Voldemort. 

Pippin