How does Johnson defend Shakespeare's mixing of tragic and comic elements?

Samuel Johnson's Preface to
Shakespeare is a monumental work in the world of literary criticism, and his
explanation for Shakespeare's tendency to include tragedy in his comedies and comedy in
his tragedies demonstrates that Johnson understood and admired the bard's ability to
depict human nature in a realistic fashion.  Johnson wrote that human
nature

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“partakes of good and evil, joy and
sorrow, mingled with endless variety of proportion and innumerable modes of
combination.”

Thus,
Shakespeare's inclusion of comedic relief in his tragedies or tragic figures (Shylock in
Merchant of Venice) is simply the playwright's accurate portrayal
of life's true nature--it is a blend of hardship and
happiness, which often exists in one's life at the same time.