Why do we forget

Cascading images falling from my minds eye.
memories lost,
memories forgotten.....
surely they must be there somewhere,
just waiting to be unlocked or are they there as we never wanted them unblocked?
hidden even from ourselves....
I don't even mean to do it now as it just happens...
they are there one minute and then next they're out.. 
at what point do we start forgetting?
does it carry on and you remember less and less till you are an old git? 
or do you always have the same memories of childhood,
as far back as your shrinking memory let's you get?
why do we forget.... 
I thought with our brain we learn and have to memorize to get on in life...
so why do we forget?
it's not just bad memories but good too that get suppressed,
so why forget them when we keep the rest