couplets

I never realized how much meaning a poet could pack into a "silly" couplet. Dryden hashes Shadwell to bits, "Shadwell alone my perfect image bears,
Mature in dullness from his tender years:
Shadwell alone, of all my sons, is he
who stands in all stupidity.
This scathing poem seems unnecessary and mean. It is very far away from the cute couplets I was introduced to in grade five. Dryden uses this poetic form to express his dark feelings. In "My Last Dutchess," the form is used to express the darkness of the speaker's soul. When did the couplet become a vehicle for darkness and gloom?