Preface to Shakespeare

ACT V. SCENE i. (V. i. 3.)

My bosom's Lord sits lightly on this throne, &c.

These three lines are very gay and pleasing. But why does Shakespeare give Romeo this involuntary cheerfulness just before the extremity of unhappiness? Perhaps to shew the vanity of trusting to those uncertain and casual exaltations or depressions, which many consider as certain foretokens of good and evil.