I think they can often be found growing in close proximity, as they prefer similar soil conditions, one of which is ground that occaisionally gets disturbed, this could be the activities of man or trampling by cattle etc.
Come here look this leaf is dock,
Beside the dandelion clock.
Where'ere the stinging nettle grows
There the healing dock leaf blows
As if to show by grand design
Of mother nature all benign
Who suffers with her children's pain
And wants to make them well again
Who cannot but provide relief
With this sting �removing leaf.
Or something like that !!
I remember this poem from yonks ago ..I remember there was always a dock leaf nearby if you were stung by nettles. But..the trick was to crush the leaf and put the sap on the stings.