Why do buffer systems use either a weak acid or a weak base?

Buffers are aqueous solutions that have a highly stable pH. 
Concentrations of [H3O] (acid) as well as [OH] affect the overall pH. In buffers, you have combinations of either a strong acid and a weak base, or a strong base and a weak acid. 

Strong acids and bases dissociate completely while weak acids and bases only partially dissociate. If you have a combination of both strong acid and base, they dissociate completely, dramatically changing the pH. However, a combination of one strong acid/base with  a weak base/acid, allows for a relatively stable pH level. 

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