In Shakespeare's Romeo and Juliet, excess of emotion is one of Romeo's character flaws. He acts quickly and impulsively; his actions are not thought out thoroughly, as epitomized by Romeo's suicide upon discovering Juliet's apparent death. His flaws of rash actions and excessive emotion prove fatal.
Romeo is also shown to fall in love extremely quickly and easily, symbolizing his excesses of emotion. In Act I Romeo was in love with Rosaline and couldn’t stop thinking about her, pouring an excess of emotion into his speech. Consider, for example, when he says,
"Love is a smoke made with the fume of sighs;
Being purg’d, a fire sparkling in lovers’ ayes;
Being vex’d, a sea nourish’d with lovers’ tears.
What is it else? A madness most discreet..." (I, i, 188-191)
However, upon later seeing Juliet he claims that “Beauty too rich for use, for earth too dear. So shows a snowy dove trooping with crows as yonder lady o'er her fellows shows.” This demonstrates how quickly his emotions change, particularly in relation to romance.
His overpowering emotion is further shown when he kills Tybalt to avenge Mercutio's death—his grief and anger turns into violence very quickly, showing his impassioned nature.
The reason for Romeo's overemotional character may be the fact he was created to resemble a Petrarchan lover, which had become a cliche by the time Shakespeare wrote Romeo and Juliet (around 1595). Petrarch was a Renaissance poet known for his poems where he poured his heart out over unattainable love.