One effect of love portrayed by Shakespeare is that love drives away all rational thought. We especially see this as being true for both Romeo and Juliet. Romeo exhibits a lack of rational thought when we see him having fallen in love with Rosaline in the very first scene of the play. He is seen staying awake all night long pining for her and allowing himself to feel brokenhearted and miserable. Romeo's cousin, Benvolio, gives him good advice, telling him to forget about her, when he says, "Be rul'd by me: forget to think of her" by opening his mind up to think of other women instead (I.i.227, 229-230). However, Romeo's response is that he can't possibly learn how to "forget to think," and that any other beautiful woman could hardly compare to Rosaline (228). Therefore, he continues to allow himself to feel miserable until he meets Juliet.
Even when Romeo meets Juliet, he continues to allow himself to be governed by his emotions rather than by rational thought. We particularly see this when he persuades Juliet to marry him so hastily. Juliet, rightly so, feels that vowing love to each other so soon is a rash idea, as we see in her lines:
Although I joy in thee,
I have no joy of this contract to-night.
It is too rash, to unadvis'd, too sudden (II.ii.122-124)
However, even Juliet allows Romeo's emotions to persuade her own rather than allowing herself to be led by rational thought.
As Shakespeare shows by the end of the play, allowing themselves to be governed by rash, passionate, violent emotions rather than rational thought is one element that is responsible for their early deaths.