Romeo's behavior throughout Romeo and Juliet is marked by impulsivity. At the beginning of the play, he is besotted with Rosaline, an unseen character, but quickly falls out of love with her after he glimpses Juliet. Their love affair and marriage are hasty, and Romeo continues to act impulsively.
At the end of the play, he has been exiled from Verona for killing Tybalt. In Act V, Balthasar brings him the (false) news that Juliet is dead, and Romeo immediately sets into motion plans to return to Verona. Balthasar does not know what Romeo's plans are, but he notices that Romeo is acting in an irrational way and says to him, "I do beseech you, sir, have patience./Your looks are pale and wild and do import/Some misadventure" (V.1.28-30). In other words, Balthasar sees that Romeo is becoming unhinged, and Balthasar begs (or "beseeches") him to be patient because Romeo seems to be heading towards trouble. Romeo answers, "Tush, thou art deceived. Leave me, and do the thing I bid thee do" (V.1.31-32). Romeo does not heed Balthasar and ignores any advice to slow down.
Romeo then goes to an apothecary, or druggist, to buy poison. The apothecary does not want to sell Romeo the poison he asks for and says, "Such mortal drugs I have, but Mantua’s law/Is death to any he that utters them" (V.1.70-71). Romeo answers him, "The world is not thy friend, nor the world’s law./The world affords no law to make thee rich./Then be not poor, but break it, and take this" (V.1.76-78). Romeo knows the apothecary is poor and plays on his poverty to convince him to sell Romeo the drugs that will kill him.
Had Romeo been less impulsive, he might have still been alive when Juliet woke up from the drug that she had taken. It is in part his heedless impulsivity that drives the action towards tragedy.